you should get a 2nd base online as soon as you can afford it. reasons to do it ASAP are hangar space and radar coverage; having more hangars available means you can run gimmick craft (BIKES! Little Bird!) to farm glamour (and thus score) or underwater craft. it also allows for greater coverage with slow craft (ex. harvester, convoy) and range-limited craft (ex. Buckaroo). notably, most aircars have trouble hitting the zones antipodal to your base; europe vs australia for example.
if you're not getting a 2nd base online ASAP, instead do so as soon as you research Alchemy and can construct a Still. This will allow you to get X-Grog production going for-profit which is the very first step of your economy snowballing. a third base can be spun up for prisoner processing & mass training, a fourth for plantations, and so on.
if you're not expanding once you get Stills online, you should absolutely, definitely, for sure be expanding once your airgame comes online and radar/fighter coverage become a consideration. this can be as early as Contacts: Car Thieves or Contacts: Blackmarch to get an aircar-alike going on.
the only limiting factor other than finance is being able to adequately garrison the bases. ideally you should be able to construct dojo/onsen/etc. to train up your recruits, however you can also make do with 'retiring' experienced units from your main deployment team to bulk out the secondary bases. either way, they're all going to need guns, melee weapons, and armor, all of which are improved by further research and/or manufacture time.