I have a number of sub-panels with a 50 amp breaker for a 50 amp RV outlet being fed by a two pole 100 amp main service 120/240, I want to know how many 50 amp outlets I can safely install, will I need to upgrade the main service panel to accommodate more 50 amp RV outlets?

Define "Number" of secondary panelboards, please.

Also, define the number of RVs plugged in at one time and the average amperage used by each RV.

If this is a RV park, you need to contact an electrical contractor to come in and review the site and size the electrical distribution system.

There is a entire section, Article 551 of the National Electric Code, dedicated for RV parks.

You can literally install as many receptacles as you want.

TK's answer is the only correct one. This is NOT a DIY project.