Do all the ZIP files need the same password?

No, but it helps. If they share a password, solving one often unlocks the rest instantly. Different passwords are attempted per file from your hints.

Is there a limit on how many I can add?

You can add several at a time; very large batches are processed in turn to stay within fair-use limits on shared hosting.

They’re encrypted and visible only to you in your account, so you don’t have to remember them again.

When you have several password-protected ZIP files — often all created with the same password — unlocking them one at a time is slow.

With a free account you can upload them together and recover them as a batch, reusing what you know across every archive.