6/3 Wire to a 100 amp sub panel, how many breakers?
If you’re thinking about twin breakers, it’s because you got too small a panel. Stop right now before you sink any more work into it, and get a much bigger panel.  Spaces are dirt cheap right now, and it’s one place you should splurge.
And it’s fine for the subpanel busing to be much larger than the feed breaker.  It’s a safety margin, 130 mph tires are safer than 85 mph tires even if you never drive 85 mph.
Your ability to use twin breaker depends on your local Electrical Code and any requirements it has for AFCI on any breaker or GFCI on 2-pole breakers.  Those are not available in double-stuff breakers, so you must plan a full space for each of those circuits.  Nowadays that will be most circuits, so I advise to just forget about twins.
Since you will certainly never use all circuits at once at full power, you can put quite a lot in a panel. Having 16 120V circuits would not be unreasonable.