There is a lot of information on these subjects on this forum. Try starting here
for some DMD basics, but have a good look around.
There is a practical limit on how many DMDs you can drive with a single Uno - and it is a lot less than 8!! Also depends on what else you want you CPU to do. Better to have a master CPU which talks to several slaves each driving about 4 DMDs. Stryker (a user on this forum) has used this principle and there is more information on this forum.
There is no need to have separate power supplies, but your supply must be capable of supplying enough current for the full DMD load (which is quite high depending on the maximum number of LEDs lit). If you do use separate power supplies, you must common the negative rails.