The objectively correct answer is "enough". Overframe is pretty bad for builds, but forma optimizing button is really nice so I recommend

if i don't intend to use it, zero including potatoes (i usually consider these to be mastery fodder, though i usually don't destroy such weapons). if i like it, one or two at least to fill out all the slots well (these also get a potato). if i intend to use it a lot, as many as it needs to do what i want which is usually 5+. my sevagoth build has like... 13ish?+ normal forma and 3 umbral forma across his three moddable sections. HE IS A VERY HUNGRY BOY AND I LOVE HIM MUCH.

so, really, as much as you want to put in, really. unless i want a super specific build, i will usually opt to put in forma on the more common polarities (V aka madurai, and ~ aka naramon) instead of a rarer one (like D aka vazarin, or = aka zenurik) assuming i have two mods of the same cost but different polarities in a build. i do this so that if i change my mind about the build later, it's highly likely i can switch without needing to reforma the thing (the umbral mods make this REALLY annoying to achieve, though, cuz they're so expensive and so is their forma)

Entirely dependant on the weapon/frame. I basically just create the build I want, figure out how many forma is needed after which I decide if it's worth the investment. For me the key part is to have the build in your head (or just make it on overframe I guess) before you start.

I have very few things with 7-8 forma though since most builds are finished before that, and too many forma can be limiting if you want a alternate build (changing elements on a weapon for instance or a different focus on a frame build), the only thing I have that is fully formad is I think the trumna.

At this stage though I don't really mind just burning a forma for random stuff, and sp circuit has caused my forma usage to increase drastically since if I scout a weapon that could be good but that I never really played outside of leveling or that has a really old build I might as well drop a potato/arcane adapter/forma into it if it can benefit from them. I found a few fun weapons that I'd just leveled and moved on like that.

any forma will increase the power level of something. so I guess you can stop where said equipment feels good. simulacrum helps.

of course if you're at the endgame, farming platinum like me, you can forma without a second thought. being MR 30 and above you don't even feel the impact of resetting its rank

It always depends on which mods you're going to use. If you're planning on incorporating rivens, primed mods or even Umbra mods in your builds (and you'll eventually want to), you'll need considerably more Forma than if you didn't.

7 or 8 forma is a perfectly normal number for a piece of gear that's modded to its max.

Most of the time I get the desired build at 3-5 Formas. Then a WF youtuber I trust drop a new good build and the forma count goes brrr... Just kidding, of course, most of the time you can get enough flexibility to change a few things unless you really go into energy-heavy riven mod build

This is kind of a bullshit answer, but "as many as it takes".

I think I have 4-5 forma on some weapons^1^, but I haven't done more than 2-3 on a frame. Some frames don't need any at all to fit the build I want.

^1^ Excluding tenet/kuva weapons, those I always put in 5 since it's required to get the XP out of them. I often find myself debating what to do with that last forma needed, since at that point the build is rather tight.

I think the need for more forma for frames probably comes with umbral builds, since umbra forma is scarce and the mods still use a ton of drain even with matching umbra polarities.

For weapons it's just "put in V polarity until it fits", haha.

if i really like them? 9. if i want to make them usable? 3-4

As many as it needs. I have a very few weapons with 6+ forma.

As many times as required to put on the build I want to put on. That was especially problematic before umbra forma if I wanted to use two umbral mods, and it's problematic today with ranged weapons because they don't have stances/auras to increase mod capacity, and the primed & galvanized mods cost a lot of capacity.

The only problem is that it also limits the builds I can put on because of conflicting polarities.

However many it needs.

If I'm dealing with a weapon with riven, I might go for 5-7 so that I get some flexibility. Usually up to 4 for non-riven.

Frames is a more by-use basis. I generally play mostly with prime frames so I will always consider an aura forma, I have a bunch. Then I just start using the frame and as I notice things I want to change to make the frame better, until I stop noticing what could be better. Typically I end up with forma'ing frames up to 3 times, the most extreme being.. 7? I think I made a mistake on that one at some point and ended up undoing it, but I'm not sure.

Generally 3 to 5 Forma on most things. It's rare that it'll need more to be able to use the best build I could put on it even with 8 Forma.

Most of my builds only have two or three except the things I really wanted to invest some time in leveling back up.

Frames 1-2 tops, i pretty much only use prime frames tho and they come pre-loaded with 3-4 polaritys.

Weapons similar deal, depends on base polaritys, but now that i have galv mods its like 5-6 polaritys needed on the weapons, especially with rivens or exilus if needed

How many it's necessary.

usually 1-2 forma for a wep/frame is fine, most of the time weapons and frames when orokin'd have enough capacity for a decent build if i REALLY want it to be comically good, 3-5 forma

for frames, mains will typically have 6+ forma, my limbo has 8 and my yareli has 9. other frames you don't use very often might have just a single, or 2-4. exalted weapon frames will likely use 5-10 total, and sevagoth needs at minimum 13.

for weapons, favorites will, again, have 6+ forma. kuva/tenets get 5, no more since 40 capacity, and no less because of the extra mastery (don't go full 5 if you don't have a ton of forma though). weapons you don't use often may have 1-4. melees typically use less forma due to the extra capacity gained with stance mods.

Eh, probably 2-3 on average. For my full on loadouts though 7-8 isn't really uncommon. Frames tend to need less overall but most of my preferred ones I've ended up flip flopping a bunch.

(Also LR2, so at this point I am just building everything out lol)