How much volume is in my fish tank and how many fish can I keep ?

I just got a second hand fish tank I want to set up. Its approx 28 inches (65cms) wide, 12 Inches (30cm) Deep and maybe 15 inches (40cm) high. How much volume does that work out to ? I am thinking of a mixed tank, some guppies, some tetras and maybe a couple of bristle nosed cat fish ? Any ideas of how many I can keep in a tank that size ?