Lemmy itself doesn’t keep a history (as far as I know), but the active users are available through the public API so there are sites that track it for instances.

Here’s one I just searched up: https://lemmy.fediverse.observer/lemmy.nz

In the Charts section it shows active users (at least one action - post, comment, vote - in the last month or in the last 6 months). It’s worth mentioning Lemmy v0.19 was the first to count voting as active users, before that it was only those who commented or posted, so the general increase in the number of active users on lemmy can probably be attributed to how they are counted (as various instances updated to 0.19) and not to an actual increase.

I do note ours shows a little drop recently, but also note that this is users whose accounts are on our instance. Lemmy.nz has about 800 users but /c/newzealand has about 1.5k subscribers and 2k users active on here in the last 6 months, so the stats are a bit misleading because they don’t count people who participate here but their accounts are on other instances.