I would say three. That is just my personal guestimate. I am only familiar with comets and common GF. There MAY be some fancy GF that could live 4 to that tank, but I really don't know. While some fancy GF might Look like they have more room, they may produce just as much waste etc.
I have read 20 gallons minimum /1st GF and 10 gals /ea thereafter. That would say 4/50Gal. BUT Looking at my 55G, I can see that 4 fullgrown GF would not live well in there.
The more I think of it, Imagining full grown GF of any type, I realy wouldn't try more than 3
If You had 3 GF in there You could use plants and decor to make the tank look fuller. anacharis is very easy to grow IME and the GF eat it.
You could put some white cloud minnows in there if you don't mind them being eaten. A small colony of white clouds would be less bioload than one more GF, make the tank more interesting And provide the GF with some live food.(because they have a lot of babies) With good maintenence I bet you could have the GF for many years, and that will more than make up for the tank looking a little empty to you when they are small.
good luck!