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)

depends on what you're going for, but crit damage is always a solid choice on melee weapons with good crit chance. elemental and faction (for whichever one you HATE most) damage are always good choices (elemental is not great for heavy attack hate builds, tho, cuz the bleed doesn't get anything from it). depending on the weapon and your intent for it range (good for hitting more things and thus eking out more damage and/or combo gains) and starting combo can also be very good (20+ combo means you always have the 2x multiplier, great for purely heavy attack builds and can let you drop corrupt charge if you were already using it). combo duration can also be very good if you're going for a max combo build since the other combo duration mods don't offer much power, if any, on their own. similarly, heavy attack efficiency is EXTREMELY powerful if you want to do a max combo build that also uses heavy attacks regularly since, iirc, there are only two mods which give heavy attack efficiency (one gives electric damage, too, so that one's solid, but the other is only heavy attack efficiency), at 90% (the cap) it only takes one multiplier out of the total you have (which is actually really easy to get back since each level is about 20ish combo count).

so really it depends on what you want to do with it or how many things you want to be able to do with it. for hate specifically, crit damage should be one of the choices at least and the rest sorted out based on your luck and/or desires.