I am now constructing something with 30 servos (each one will control an individual joint separately, so I guess I can't get away from this), how can I determine how much current do I need, I have a 5V 6A power supply, will that be working just fine? Many thanks in advance!
You'll need to look up for your particular model of servo what it is rated for.
From this random datasheet I got values of about 5mA while idle and 150mA while running. Your 6A supply will be fine (if your servos don't draw significantly more than these) even if you run them all at once (which in practice, you might not).
Note that if you stall the servos they will draw more current, but it seems unlikely that you'd stall all of them at once.