Checked that my project had 5816 files in the animations folder so figured I'd try this too. Adding an animation with 200 frames caused preview to fail in Chrome and NW.js, so I can confirm that it breaks at 6000 files.
Chrome seems to have a purpose built constraint: https://codereview.chromium.org/18541
This doesn't affect me right at this moment, but will be hitting that limit pretty soon.
Since it seems this bug has been closed as "won't fix" before, here's some points of reference:
This is nethack assets:
There's 800 unique items without animation.
It's not at all uncommon for a commercial game to have more than 6000 total animation frames.
Single character in street fighter: https://orig00.deviantart.net/ff4a/f/2011/223/a/1/cody_sprite_sheet_from_sfa3_by_anis91-d466t6q.png
Also the Space Blaster demo provided with Construct has 274 files in the animations folder, that's 4.5% of the maximum.