How many sc gengar does anyone need?

Unless you are trying to solo Claydol without weather boosts, 1 is more than enough.

Up to three for optimal use (Alakazam Confusion to get ahead of the clock) or two (Clydol). Note that I used the word "optimal" as in most efficient during raids. However, three wouldn't be worth my dust. Two, maybe. One, yes.

Other than that, i wouldn't recommend investing too much in a glass canon for other type of raids.

If you like soloing T3 raid 2 gengars SC/SB will hellp in Alakazam and Claydol raids..
For group raids, you can use another mons like Tyranitar against mewto. You can use Gengar if you want to trio mewto as a time shaving glass cannon option if you want.
3 will also help, but must consider the cost of stardust for poweing it up.
I only use 2 now. one at level 39, one at level 35. You dont need to get that high for psycho cut alakazam, but need it for the confusion one

I think you should TM to Shadow Ball at some point, never know when that may become legacy.
As for power ups, I agree with the previous. Make them side projects if you’re not trying to spend too much.
For Claydol solos two is good, three is a bit overkill for glass cannons. Two should be good for Alakazam as well.

I have zero SC/SB Gengar.

Only have one low level SC Gengar with 5 attack IV... Is it worth powering up or should I invest in my H/SB IV96 lvl 32 one?

My gut feeling tells me a SC/SB gengar is overrated and max a few % stronger than a H/SB Gengar. I'm not enough of a hardcore POGO player to bother doing the math.