Threerounds, there aren't any 10 shot 9mm magazines, but they are made in .38 super. Perhaps not too critical for plinking or even self defense, but that extra round can certainly be worth having in competitions such as IPSC or IDPA.
Also, it's always good to know that certain substitutes can be used in a pinch. For example, until I could locate some actual 10mm mags for my Delta Elite, I tried out Wilson 8rd. .45 mags. They hold 9rds. of 10mm and seem to function perfectly, which let me get to shooting the gun in a match a couple weeks sooner.