a ten gal is a great size, and if they don't want to worry about tons of baby guppies then just get some pretty males, they won' really fight if here are no females present. other wise it is standard 1 male to 2 females.
i woudl say only 2 trios but they would have to remove fry, or 6-8 males if that is the only fish in the tank.
if it is a beginner with a cycled tank.
I have kept guppies in more crowed tanks but with tons of live plants, and always did my water changes, and checked my water parameters. this was always temporary until i found homes for all the babies. We all know how unfair poor water and cramped conditions are to fish.