Looks like it's a customized Beretta 92 or M9(not much diffrence, really). It's hard to see, but it looks like it says "9mm SWORD CUTLASS" on the slide, thus it'd be a 92 if chambered for 9mm. There's a couple mods that allready introduce the Beretta, so this could be dones as a simple reskin of an existing model. I'd suggest contacting the authors of one of the Beretta mods and asking if they'd be willing to do a reskined version. Worst that'll happen is they'll say no, so might as well take a shot(no pun intended).
As for ammo capacity, the 9mm version has 15 round magazines as standard, with aftermarket ones offering 17 or 18(fits flush, as in pic) or 20(would protrude slightly) rounds. If, for simplicitys sake, you want to use 10mm, 12 rounds is a good compromise for a flush-fit magazine(IRL, there's a Beretta 96 chambered in .40 S&W, which is less powerful than 10mm Auto, but has the exact same diamater. It holds 11 rounds standard, though as with the 92 there's after-market mags with larger capacities).