When creating a button for the UI, how do I get the current record? I'm doing currentRecord.get(), but it is still returning -1 for sublist item count

more context needed

You will need client script. Call the client script function from the button.

I'm going to guess you are in View mode not Edit mode & that is why you are getting

-1

instead of what you are expecting in my experience this is just a limitation of using currentRecord like that - must be in Edit mode instead

correct. I ended up having to do a record.load since currentRecord.get() wasn't working