To further clarify:
The current Solaris Arena maps released with MWLL 0.5.0 were designed for a maximum of 16 players. Maps: SA_Jungle, SA_Glory, SA_HelsGate
The mode itself does not impose a player count limitation, but with more than 16 players on these maps there will be occasions on which you cannot spawn due to the 'Mech hangar being occupied by another player and generally higher level of chaos on the battlefield due to the player density.
This player count suggestion is for the initial release maps, additional maps that will be produced by the MWLL team or by community members may have differing suggested player counts. In the case of MWLL authored maps they will be accompanied by a suggested player count, I would suggest that community mappers also provide this information.
Apologies for the confusion.