Just open Skirmish menu and then hold Shift and 8, then release them and press X
 
 
Another info about it:
 
1 = i ; 2 = ii ; 3 = iii ; 4 = iv ; 5 = v ; 6 = vi ; 7 = vii ; 8 = viii ; 9 = ix ; 10 = x
 
 

Have a nice day and good luck!

 
 

 
 
I hope you enjoy the Guide we share about Total Annihilation – How to Add 10 players in Skirmish; if you think we forget to add or we should add more information, please let us know via commenting below! See you soon!