from the 20 pin or 24 pin plug, cut the green wire and one black wire. solder or wirecap fit the green and black wire together. note your powersupply will be voided if you do this. the green wire is a signal cable too the motherboard it tells the powersupply a motherboard is connected.
next plug your fan onto one of the molex connectors. plug in powersupply and switch on. note fan should spin up. if fan spins up your good too go. if fan does not spin up your powersupply could be faulty or be of a newer type.
as for the supplys themselves, a 200w supply will power roughly 4-5 fans maybe more
a 400w supply will power over 10 fans.
do not exceed the stated amps on your 12v lines.
check your pc fans, there are some on the market with small controlers fitted, these allow you to control fan speed, cmf output and noise.
you can see a whole modified version here
https://www.rollitup.org/do-yourself/99581-computer-power-supply-mod-12v.html
Tamzi