Technically, there is an infinite amount of beatmaps as you can create your own beatmaps using the editor.
You won't run out of beatmaps any time soon, this game has more than you will ever play just in the ranked section alone, not even talking of pending / graveyard maps.
Yeah I know that there is an infinite amount of maps since everyone can create one whenever he wants but approximately like at the beginning of the year how many maps was "downloadable" on the osu website ?