It depends how you define "use". People who once logged in into the PGC? People who have statistics in their profile? People who use this QA forum? People who donated/became paying members?
The first estimation can be found in this QA site: Here is around 1726 users, but this is clearly not a number of PGC users, but it can be considered as "fairly active users":
https://project-gc.com/qa/?qa=users&start=1726
Another estimation can come from the PGC Points. We do not see the tail of the users, since the list is limited to top 1000, but top thousand have Ruby PGC Badge and you can find fairly many users with Bronze one, which means there are more that 25 000 users who at least authenticated to PGC.
https://project-gc.com/Home/PGCPoints?chartpage=34
ganja1447 will probably have better estimations, but this is what we can observe from the "public sources".