[@Enkryption] Every servant is generally powered by Chaldea, or some sort of other generator in Fate Gudao (or fujimaru if you really prefer it doesn't really matter) is an overall average mage if not below so and isn't capable of handling the general amount of gods. Every master in Fate/Grand Order was meant to summon servants through Mash and Gudao does so as well. Gudao is required as proper human history's master, and so was Team A aka the Crypters. However Mash is not in this as I don't feel I could write her well enough replacing her with a summon generator built by Archimedes by some sort of mcguffin technique or what have you. To answer the question flat out though masters are anchors, and individuals who can control, and make sure the servant in question doesn't go rogue in the singularity, or otherwise die in combat. The master summon the servants and keep them there through mana, and the bond through the summoning process. They serve as a way of having someone there in case the servant dies, or the hand of an actual living person is needed.

As such I could use Leon to build a all around master candidate, or Rezod's but they would still be limited to three command seals, and likewise be built around lets say Leon, as a master candidate. But I feel that would be way to Mary Sue esk, and I have no intention of making Leon into such a character. If a player wants to open up their master to having more than one servant that's their choice HOWEVER if they bail that servant is left with no master and thus no anchor and thus they can't stay situated in physical form for long. If say Leon died Mordred would not be able to fight on for long as she's not an archer and has no access to independent action. And those with IA who aren't immune from this because unless they're gilgamesh with a ridiculously high level of it they'll probably just wither away.

If you're looking for anything more on that I can't say because i'm not a grand master on the excessively large amount of lore the nasuverse has. 

Edit: Also servants by definition are incapable of having command seals unless they're rulers overseeing a grail war. The only ones i know of that have them are Amakusa, and Jeanne both rulers one of which took the seals from pre-existing masters.The other being Medea who took them from Shiro using a weapon specifically meant to break rules, or Shiro gave them to her to save rin or something don't remember.