Is there a limit to how many videos I can upload?

No, go to town: Upload as many as you like!

Just consider that we're running as a free service with limited server power:
Please mind other users, when you are planning to upload a large number of videos at once.

Other users will be behind you videos in the conversion cue, and will have to wait for all of your videos to be done.

What's the best video format to upload?

There is no best format - all formats get converted equally. Of course you should upload your quality to get the best result.

On syncsketch playground, we are running a fairly strong compression, so that videos cache and respond fast in interaction.

We are thinking of giving users the option to set their compression level during upload, so that they can optimize playback vs. image quality.