i'm inclined to think that they are enarly 1k, they were 500 in 2014, they probably doubled in this 1 year and half
and if you count even those that died at launch or were a simply remak, they far exceed that amount
There are so many new ones coming out each week, on the other hand many are delisted on exchanges as well. It's been a revolving door, but the trend is that a growing number of different alt coins remain.