Some new details regarding the options for player numbers have been announced via the official Metal Gear Online Twitter account.
I have some good news this morning for fans of custom matches and additional options. After some deliberation, and thanks to your input, we have implemented a custom match option that allows for additional player numbers. When playing a custom match, you can select 1v1, 2v2, 3v3, 4v4, 5v5, 6v6, 7v7, and 8v8 # of players.
A couple of questions that were asked on Twitter were answered as well, providing some further details. The setting for normal matches is 6v6 or 8v8. As you can see, the custom match options only offer even numbers of players on each side, as that is what the game is designed around. Furthermore, the game only supports real players, so there are no options for bots at this moment.
Source: Metal Gear Online Twitter