I got around 100 now, the most I've had is 200+.
I know some real money makers that have like 1,000+ domains.
However I see they don't make money from most domains.
So I think 20-40 domains is enough unless you are making money from each one then 1,000+ is ok.
Most of us including myself make the mistake of buyin 10+ domains and never finishing the required work on them all. Probably better idea to buy 1-3 domains and buy new ones when you've finished builsing sites on the existing domains.