That means that the gun's "capacity" is ten rounds. In other words, you can buy 20 rounds every four months.
The interesting question is what constitutes "ammunition for an assault weapon." Given that there are "non-assault weapons" in every caliber used by "assault weapons," one can only assume they would use the SAFE Act's registry of assault weapons to determine who can buy one box of .223 every four months and who can buy as much as he/she wants. Ammunition sellers then would have to be given access to that registry -- the one that we were told was going to be so benign -- to use as a tool in denying or limiting sales. Such a scenario is highly unlikely, since the background-check-for-ammo provision of the SAFE Act has already been shelved as an expensive boondoggle.
When they first floated this piece of dreck, the intention was to apply it to all firearms. Five-shot snubnose? You get ten rounds. Double-barreled shotgun? You get four. Save up your allotment for two years and you'll have almost enough to shoot one round of trap or skeet.
I can't imagine that the people who dream this stuff up actually think it is in any way reasonable or even potentially effective. They're just casting around for anything they can think of to fuck with gun owners.