This comparison is pretty much meaningless imo but in case you want to continue it's better to compare similar branches. For Dash corresponding Bitcoin branch is 0.10, not master.

So difference is somewhere around 2100 commits currently
Thank you for the correction. That seems an impressive number, and indeed somewhat an indication of a lot of development work being done that is independent of Bitcoin. If you look back in the actual commits there were indeed many commits for clean ups, tweaks, etc., especially during the summer (around the time of the version 12 release), so the 2000 number seems to pass a quick reality check.
Are you no longer merging new Bitcoin commits, or is it planned to freeze for a while and then catch up at some point?
Yep, the idea was to freeze v12.0 in some stable state (which we hopefully achieved) and release new fixes and improvements (as well as merge latest commits from Bitcoin 0.10) in v12.1 somewhere before v13. v12.1 still require more work to be done (still having issues with locks sometimes) and community testing before launch for sure (few important things changed there including tx creation and budgeting rpc) so not going to happen very soon imo. Moreover we are mostly concentrated on developing ideas and implementation of v13 now.