A Basic ranged weapon is allowed one mod, plus one more for each level of WhizKid.
An Advanced weapon is allowed four mods, plus four more for each level of WhizKid. A maximum of level 3 applies to each individual mod. For example D3A3 etc.
Unique, Special, and Melee weapons are not allowed any mods. Note the BFG is a Basic ranged weapon and mods are permitted. Personally I like magazine for the first mod on this one.
WhizKid also permits multiple Armor and Boot mods to be applied at the same time, max level 1 for each mod.