You can have more, just search on these forums, Google is your friend. I remember not long ago there was a conversation about this and someone managed to have more titans at once. Should not be hard to find.

Well if you want to have a go at trying to mod it in, this is the thread where we discussed it and found a way to do it. However, it is not just changing a few lines in the files, in requires you to learn how to make entirely new abilities, so it may not be the easiest thing to start out modding.

As for existing mods, to my knowledge Sins of a Galactic Empire is the only one currently using this feature to allow multiple titans.

Yes, you can also do this too, but that's basically cheat mode. 

Not if you mod them in. You can set it up so the abilities have resource costs you have to pay, have a delay to simulate the construction time etc. In the dev.exe you can just spawn in any ship you like, kill anything you like, give yourself unlimited resources, make your ships invulnerable etc at the push of a button.

Buildings all start with PLANETMODULE.

You have to make a new ability and put it on something you can build in game, like the titan foundry. You only need to change the player file if you want to say give the Advent a TEC titan.

so i need to create an ABILITY entity file... and latch it on the titan foundries am i correct?

Hmm.... yes however i am planning to do it on Advent Rebels instead... how does that work?

http://woeaintme.wix.com/woeaintmods#!titans/c1tu2

Shameless self promotion... Download that and have a look at what was done. Reverse engineer it as you wish.

or you could just add them in on the galaxyscenario for all factions so its fair and balanced, just give your self as many as you give the other factions, then if they are all destroyed then you and the ai are reduced to factory single builds. Its how I did my mod.