The straight answer is in your stats - 1217 rounds from signing in.
Alas, you'd want to know the no. of Legend rounds - more difficult.
1) make a note about the total no. when you turn legend and evaluate the difference every now and then - probably not applicable.
2) count the legend rounds in the score history if there are still some TM rounds visible - too late for you.
3) if you know when you turned Legend, make a guess from the no. of weekly or monthly rounds you play.
4) note score and average after every ranked round for a while and calculate the (approx.) no, of Legend rounds from that,
5) wait until you reach saturation, visible when bad rounds don't push the ave. up again, but mediocre rounds make it fall like a stone.