It says that about 1,300 people have joined the jam. How does that number usually compare to how many people really enter/finish games for this? Anything even close to that possible 1.3k would be a huge jump compared to previous jams.

It depends, but I wouldn't be surprised if this gets divided by two

Of the 1709 people that joined the jam last year, there were 366 submissions. So if we go by that ratio, we can expect around 300 submissions.

Yeah, keep in mind people still join throughout the jam, and especially at the end in order to upload. So it's still too soon to tell! :)

It'll be a lot, either way.