In your estimation, how many alien civilisations out of 100 have invented the zipper?

It’s of such practical use that something “zipper-like” is almost a guarantee provided these civilizations are like ours.

Are velcro, or a series of buttons, or a series of holes with a string through them, or a sticky strip, zipper-like enough to be considered the same invention?

No, it needs to be THE zipper.

Then likely zero. Odds are humans would not even reinvent it.

Zero. Any sufficiently advance civilisation would have dispensed with the zipper long ago and replaced it with something that works much better and doesn’t get stuck.

Who says aliens wear clothes?

Zippers are used for all kinds of things. Like body bags that soldiers return in from Iraq.

0. Tape and/or elastic are all that is required.

Gilsnuxians secrete slime that obviates the need for adhesive technologies.