Does reddit number of comments and post per day keeps increasing or it's declining and reddit has it's best days behind? Do we have any data on it?

I’d say it’s actual human user base is on the decline

We tend to underestimate how large reddit is and also tend to overestimate how much the quality of the communities we participate in affect the average reddit user.

https://backlinko.com/reddit-users

It grew pretty heavily in 2024, there are other places that collect stats too, but in most ways that you can easily measure, it's moving in that direction. 2023 did have a small blip in comments posted per year, but it got back on track fairly quick.