Whenever I needed a pouch, I've been sourcing i 2nd hand for cheapest price I could find, not caring about the looks. meaning my loadout looks a darn mess.
today at my belt, since I swapped for hicaps: double pistol mag pouch (multicam, contains a pistol mag and a speedloader), pistol pouch (black, more a pouch than a holster), and utility pouch (tan, using the molle on outside to attach SAK, BFG loading rods and BFG pins on a carbiner, inside is blanks and anything small that I think I might need).
a BFG in each front trouser pocket, spare mag in knee pocket, smaller packet of blanks in chest pocket.. set to go!
well, might aswell me mention that I use a 1point sling then aswell.. cb.
before i swapped you could add a dump pouch, extra dual pistol mag (1 mag 1 battery), bottle pouch for bottle of bbs, vest with 6 pouches that takes 3 mags each, pouches with speedloaders and a pouch for the leatherman.
wow, there ya go!
edit: yea i wrote without reading op
ANSWERING OP: I'd say for beginner, get a high cap, maybe two, noone will look down at you in the field, keep the spare one in your pocket and a spare battery in the other pocket, and you will find out yourself when and if you need pouches and buy them as you go along. Keep your cash for walkons fees, transport, ammo, upgrades