Is there a way to set the seed for the random number generator in Apex? And if so; which function do I use for it?
It likely isn't possible to seed the RNG in Apex. If you need a repeatable sequence of random numbers, you'll have to implement a seeded pseudo random number generator yourself.
On the Apex platform, I'm sure they have a huge source of entropy available to generate random numbers, and there's no need for you to seed the generator.