just speculation but based on other games it will probably be 1 to 3 tanks at any given time. Could even be up to a 4th but i doubt that will be very common.

Sounds like you are worried that you wont be able to gear your PLD up. For my FC I have set up the rules that you declare a main job, lets say PLD. We are full on tanks so I ask you to DPS. Though you are running a DPS job we will have it set up so you can still roll for the PLD gear as it is your main job. We will also rotate tanks through, if their skill permits, to allow everyone to tank. If a FC isn't willing to work with everyone then it isn't one worth being in.