See
Collection Guide 2: Quantities (Fully Updated). But honestly don't really worry about this for what you should buy as if you are seriously going to play competitively most people will let you borrow, and if playing casually you can always proxy. For what armies you want more than 4 of, I would only say the "swarm" armies such as Vipers, Zombies, and Marro Drones & Dividers; or if you are really a cheap player then play 5-6 each of the B&B ranges squads of 4th Mass, 10th Reg, and Stingers (I have seen
@
@R˙chean
play 8 squads of Stingers once at Gencon). Also note the "Firestorm" has 7-10 Fire elementals and Kurrok and is probably the unit with most copies in competitive armies, unless you are
@
Matthias Maccabeus
and want 20 Drow Chainfighters
.