Corrected entry: When Tony is shooting at Boris and he just won't die, Tony fires a total of 8 shots, the last one being Tony actually aiming down the sights. Desert Eagle magazines only hold 7 rounds.
Correction: Unless he put one in the chamber. Then there would be 7 in the magazine too, making it 8 rounds total.
Correction: Tony is a blagger who actually has a .44 Magnum, rather than a .50AE. You can see from close up shots that it has a teardrop shape safety, which means it's a Mk1 which was never chambered in 50. A .44 mag holds 8 rounds and a .357 holds 9. The really ugly "Desert Eagle .50" font on the gun is also non standard. So I reckon he has a .44 magnum that just has "desert eagle .50" written on the side. He consistently fires 8 rounds before reloading. How Avi fires about 11 shots with it in the "look in the dog" scene is a different matter.