I would say don’t try to get too big, when you get over 10 or 12 it starts to get difficult to keep everyone on the same page. Don’t assume that just because you get along or trust a certain member that everyone else in the group will feel exactly the same, you never know what long held grievances someone may be hiding.
Be sure skill sets are diversified, be sure everyone is a realist about their skills and their place. Be sure everyone either has similar moral codes or that they are at lease willing to be flexible during SHTF situations. Morality situations can bust up a group and once someone knows your groups capabilities and goals getting rid of them isn’t quite so easy. Opsec, Opsec, Opsec. Loose lips sink ships.