About 130k accounts. I think that word of mouth will prevail to get new players in to try the game. Not all of those will have bought a spellbook as it is too restricted before you do to properly try it out.
In terms of coming to the party late - I think you can still make some decent headroom by renting cards. I came late, and have just started renting yesterday. It was the best thing I have done as I can progress up though the leagues. The additional DEC earnt pays pack the rental costs back quite quickly, so it is a win-win for me.