You're still operating under the basic rules of your codex, the only thing that goes out the window is the FOC. You could only take as many lascannons are you're normally allowed to. It may be apocalypse, but you're not allowed to just ignore restrictions on gear and options or else things would get out of hand very quickly. And as a Black Templar player that technically means you're still only allowed one relic per army, but that's one you should talk to your opponents about beforehand, I'm sure most people would be willing to throw that out and allow you more considering that's pretty harsh in an apoc game.

 

Thank you for the response. I agree "with things would get out of hand very quickly".