California has that very law.  10 round magazine.
Didn't keep the two Muslim fanatics from killing  all those people in San Bernardino, though.  
Colorado has a 15 round magazine limit.  Remember the theater killer?
Connecticut has a 10 round magazine limit.  Remember Sandy Hook?
Maryland has a 10 round magazine limit.
Massachusetts has a 10 round magazine limit
New Jersey has a 15 round magazine limit.
New York has a 10 round magazine limit but you can only put 7 rounds in it unless you are at a range.

 When there was the 1994 Assault Weapons Ban, the evaluation pointed out that at that time there were "25 million to 50 million large-capacity magazines still in the U.S."

They are still out there.  

In fact, I would bet that with 310 million civilian  firearms being in the US (per the Feds,)  that there are many, many  millions more than there were in 1994.   Almost everyone that I know who owns either a long gun or a semi-automatic pistol has at least two magazines per firearm.