According to a random page I found online, pigs are typically slaughtered at 5-6 months old, so assuming a stable population, you'd expect the number of living pigs (raised for slaughter) at any time to be 42%-50% of the yearly amount, so 375,000,000 - 450,000,000 going purely off of your 900,000,000 figure, making 500,000,000 a reasonable estimate, no?