If I'm entertaining and doing it with food involved, I'm making damned sure no one doesn't get what they want and plenty of it.
Better to have leftovers than dismayed guests.
And some guests love a BBQ doggy bag.
That and I'm a leftovers chow-hound... Woof!
I'm gonna say you want to be able to serve at least four bones of the ribs per person and/or a 1/4lb of brisket.
So I'd be getting five more racks of St. Louis ribs.
Don't do brisket flats, too easy to screw up and then be short, full packers are much more forgiving.
You're going to lose at least a third of the precooked brisket weight, so you need a minimum of 7.5lbs of cooked brisket.
I'd get two 15-20lb full packer briskets and after trimming and cooking you should be good to go.
Put the beef ribs back in the freezer if you don't have enough for everyone.
Time, leave yourself lots of time, time to prep, smoke, holding time for the briskets and of course time with guests.
I'd cook the meats the day before, wrap un-sliced in foil and butcher paper and then reheat in the smoker the day of.
Push a little more smoke than TBS to get everybody's appetites peaked, then pull slice and serve.
You could also require RSVP and have guests select their meat preference in advance, easy to do now via email and texts.
This can really be a big help in knowing how much of what to prepare.