After 2000, 10 rounds max. However, you may import any and all hi-cap magazine components (including the mag body) as repair parts for mags legally owned before 2000, but it would be illegal to use these replacement parts to manufacture new hi-cap magazines. One could also have magazine parts kits (for mags NOT owned before 2000) that are assembled into mags when traveling out of state, and then disassembled into parts again before returning to California. As long as they are not used to manufacture new mags in California, it is legal, there's nothing in the penal code stating otherwise.
You can also legally use hi-cap mags you legally owned prior to 2000 in new firearms, i.e. 20 round mags in a new M1A, or 15 round mags in a new Beretta 92. A common misconception is that you can only use 10 round mags in firearms bought after 2000 in CA. There is nothing in the penal code to support this whatsoever. Many people (myself included) bought a huge amount of mags before 2000,many for guns we didnt even own yet, for this reason.
Gene Hoffman's magazine memo: http://www.hoffmang.com/firearms/DOJ...2005-11-10.pdf
This pretty much sums it up, if a seller doesnt want to sell you magazine parts after reading the DOJ memo, it's his problem. Find another seller, there are plenty out there that will take your money.
Try Lance at LAK Supply. He sells complete disassembled new magazine parts kits to Californians at a reasonable price. If he doesn't have it, he can get it for you. http://laksupply.com
Arent the SL7 mags 10 round anyway??