@rastinza I don't know of anything that does this natively but it should be easy enough to script with the mastodon.py tools.

Basic idea:
1. Set your counter to zero.
2. Call timeline() to load your home timeline.
3. Check if any timestamps predate your cutoff.
4a. If so, count how many posts do not predate the cutoff, add that to your counter, and return the counter.
4b. If not, add the full number of posts to your counter, set your maximum ID to the minimum you got this time, and return to step 2.