What is the maximum of frames for a single project?
@BasToTheMax 10K frames. But it’s absolutely not recommended to cram 10 thousand drawings into a single file.
Even commercial software will get bogged down and not function properly.
We always recommend people starting out to simply follow industry standards where possible, that is, creating one file per shot / cut / scene and then string them together in a video editor which are designed to hold many hours of video and audio footage without breaking.
Despite being an obvious remark Pencil2D is an animation software and not a video editor, and thus it’s not designed to blend well with thousands of drawings in a single document project file.
Another recommendation is to please always create backup copy files. Never ever save on the same file for more than a few times or you’ll shoot yourself in the foot when the files gets damaged by a fortuitous event (blackout, power surge, operating system instability, pets chewing power chords, etc).
This happens often with commercial software as well. File corruption is a real thing and no one is exempt from it, so the best way to prevail when one loses work, is to have backup copies lying around to fall back.
I’ll let you know that due to previous experiences, the possibility to recover a corrupted Pencil2D file is rather low in most cases, so don’t rely on it, please make backup copies to develop a habit as part of a professional animator toolset.
Say, is that 10,000 frames on the timeline or 10,000 keyframes?
@JoeyH There’s an intended timeline length “soft limit” of 10,000 thousand frames.
I say soft because you can hack this restraint and keep going for a long time. I did the test personally and I could move the playhead near the 1 millionth frame (yes 1,000,000) to add keyframes, before getting bored.
As for a limit on adding keyframes, I don’t think there is (though read the next paragraph) but if you add 10 thousand keyframes you would more than likely using Pencil2D or any other animation app, incorrectly.
That said Pencil2D’s actual technical max limit, that allows you to export something is around 65K frames and for that you’d need a super beefed up PC and having a death wish for your files 
10K is just a safety lock though we didn’t really come up with it ourselves and we don’t recommend going there either. I personally would recommend never going past 720 frames at 24fps, which is roughly 30 seconds, and even then that’s absurdly uncommon.
For example this year I had to animate a sequence shot (also called long take) for a client that lasted between 25-30 seconds and even in paid software (Adobe Animate) it was tough to work with, due to the amount of frames and layers the app would crash at least 4 times a day (and I would lose work like a champ).
It’s better to work smart and always plan separate shots in separate files, then string them in a video editor, and even if you’re going for a specific effect or statement any fluid motion shot shouldn’t last more than 10 seconds, it’s just too much work.