YSK: Subscriber count on communities only show the numbers of users subscribed from your specific instance. The real number might be much larger than you think. - eviltoast

You can use https://lemmyverse.net/ to check actual subscriber numbers.

Edit: Why YSK: New users of Lemmy can find the number low and think that a community is dead or inactive, when infact it might be a thriving place with a lot of activity.