How many alliance members can get B loot or better?
In theory, 30/30 can. B = 3.30% damage. If 30 players do the exact same damage and kill the titan, 1/30 = 3.33333%, so everybody qualifies.
I don’t know if that has ever happened, 'tho. Trying not to overshoot titan damage is quite hard to do, if you’ve never tried it. (Most people don’t have a good reason to try!)
In practice, if everybody in the alliance is capable of doing their 3.3% share in a reasonable number of hits, and tries not to overshoot, I’d guess that 25~28 people can all get a B grade or better, but I’ve never been in a full alliance where there was general effort/policy not to overshoot 3.3% titan damage, so I don’t know how well it actually works.
If you only have 15-20 members, it isn’t that hard to get everybody over the 3.3% threshold.