v1 Nov 2007, not used in public
v2 Sept 2008, ~555K SNPs
v3 Nov 20 2010, >900K SNPS, can be uploaded to FTDNA for $39, only version uploadable to GEDMATCH
v4 Nov 2013, ~570K SNPS

The link www.23andme.com/you/compare/ can tell you how many SNPs you have in your dataset (this doesn’t seem to be accessible from any of the menus, as far as I can gather). If you go to the One-to-one tab and put yourself on both sides (or anyone you are sharing genomes with, if you want to help them), the number of SNPs is shown.