NSA started releasing these numbers in 2015. In the 12 months ending August 2015 there were 9,217 unmasked. In CY17 there were 9,529 and in CY18 there were 16,721.
I think 2019 was around 10K.
Statistical Transparency Report Regarding the Use of National Security Authorities
The unmasking of political opponents is extremely unusual, and beyond the pale.
How do you know they're your political opponents until they're unmasked, and if you know who they are why bother asking for them to be unmasked?
As for why, perhaps to show - "member of the club" status (I looked at it too, my status is as high as yours"), or to keep story lines straight, or at the urging of other top ranking members, to be at equal risk, multiple possibilities.