Are you talking about getting it to work for your entire collection or a sub-set?
You can get a sub-set to work on the NUC, how many is going to be by trial and error I would have thought much on the lines of what you are doing now.
It’s hard to be precise as a lot of the growth in the database would I imagine be driven by the underlying tracks and their network of linkages. If you have a fairly obscure set of tracks with standalone artists then the increase in metadata linkages may be modest but if you had tracks with every popular session musician in the last 50 years in them then the web of interconnections is vast.
The escalation makes for a vast database, as below, and to cope with that you need a very fast processor running at high clock speeds, which you don’t get in a NUC.
Roon support may know more accurate estimates.