[quote=@Isotope]
By law, four.
[/quote]

Well, by the law of HAEL (the one true law), a max of 18 billion humanoids can inhabit a single planet. 

As a result, I will ask you to count 4 mats as the equivalent to 18 billion humanoids, or 1 mat as the equivalent 4.5 billion. 

It might seem trivial now, but I can imagine several scenarios where knowing how mat population compares to humanoid population might be important.

Also, unless you have some reason that it should be different, I'm going to ask you to follow the same general rule of "a maximum of 3 solar systems for every inhabited planet", for the sake of keeping it even. 

And while we're on the subject, do the semi-sentient aliens that serve the mats work in armies to defend the mats? Because if they do, then you should probably count that as your military for the "Military Size" and "Military Details" section of the NS.