As mentioned before, get a take-up spool at most any processing lab. The takeup is the same spool as the film is wound on originally.
120 film winds off the film spool, all the way on to the take-up spool. The 120 film even has a lickem tab on the back end to seal the roll shut until you take it to the processor. Then the spool that is left on the camera becomes the new take-up when you move it to the other side of the camera.
You say there are two settings and/or film masks for 12 and 16 exposures. That means you can shoot square 6X6cm and rectangular 6X4.5. There must be a setting on the camera for changing the format, OR perhaps the Holga has two ruby windows in which you read the numbers as you advance the film. That's the way it was done before advanced film counting and double prevention.
The take-up spool is not a real issue, as they are plentiful and would be the same for all 120. If worst comes to worst, (and it shouldn't come to this) buy an old roll of 120 expired film and strip the film off (blow the roll) to get a spool.
I don't really advocate that, as I mostly shoot expired film bought cheap off eBay and find my results have been fortunately exceptional. Find an eBay seller who sell a ton of expired film, indicates that it's been kept refrigerated, and has a couple of thousand feed backs at 100% positive. That person is not lying if they have made 2000 people happy with their product. There's a lot of old film out there waiting to be exposed and developed. And it's very reasonably priced. Usually half or less than current date film. Of course, I am not relying on my shooting for income, so I can take the risk on an occasional roll that's got color shift. And if you are shooting B/W, I have never seen the difference between expired and current date film.
And if you're using a Holga, you're after kinky results anyway.
