This histogram from Stats iOS app was shared just before FSD Beta 10.2 rollout:
View attachment 725791
Directly applying those percentages to 150k would suggest FSD Beta grew by 18k participants with 10.2 and 26k with 10.3. The relative sizes of 99 vs 100 is 1.4x, and that does roughly match up with Teslascope+TeslaFi numbers (636 on 10.2 and 1648 on 10.3.x -- 1.6x increase).
If the FSD Beta population is indeed around 44k, then since October 11th, on average there's been 3k participants added each day. Of course we can scale back the number for more "above average" scores, so discounting by a factor of 3 for ~14k participants would be an average 1k/day.