So, how much HP do you need to do simple drifting?

so long as you can get above 40 km/h, you can do it with 2HP, assuming you have a handbrake. Might not be able to maintain a drift for long, unless you bald you tires, but thats alright.

All the hp

Less weight always helps.
Example the miata, it is not the most powerful car, but with an LSD it can do some skids. For me around 140-50 hp and a car that weights no more than 1100-200kg is enough for simple skids, but lsd and stiff suspension are a must.
I am talking for dry tarmac, on rain - everything rear wheeldrived and with LSD :D

3hp works for me ;)

AE86 all you need

Got a FWD so technically not drifting. But it still goes half a roundabout

Veiksmi driftā!

I just got the back end to slide out by clutch kicking my 66 horsepower Suzuki Sierra in the rain. theres a lot more variables that come into play than power

I had a e36 316i Touring with about 100hp but u couldnt Drift on a dry road
The car was too heavy

Yeah if you want to keep replacing your clutch. It also comes down to torque and power/weight so if you can, do a lot of weight reduction :P