Best number of modules?

[ginntonic]

I usually stop building them after they reach 40 industry... What are some different strategies?

[Timerlane]

I would say no more than 3-4 Food/Industry/Science-producing Major Modules per floor(maybe 5, if conditions are ideal; if there are Silic Bulldozers, Necrodrones, or Kamikazes in play, you must keep them 'hidden' in/behind your powered chokepoints), and keep in mind how many doors are likely to be left before you decide to build more.

Obviously, the more doors remaining, the more value you get for the Module. If you build an Industry Module and then fail to recoup the extra Industry you spent building it, you're just wasted Industry. If you plan to put an Operator on that new Module, you'll get that Industry back much faster.

If building more than 3 FIS Modules, I'd usually double-up on Industry first. If you're done with Artifact research, you can always skip Science as long as you have at least the 140(-ish) you need to fully power your Level IV KIP Cannons, unless you're really spamming Active abilities and need the immediate Science to refresh them.

Amusingly, as much as Silic Zoners are the bane of Minor Module users everywhere, they will completely ignore a room with nothing but a Major Module in it(even the Elite Zoners, unlike most Elite monsters whose behaviors usually expand upon those of the 'standard' variant).

Handy semi-related tip: The number of FIS-producing Major Modules present also affects the cost ramp on Emergency Generators. If you plan to use Emergency Generators to help light the way to the Exit, plan to 'bulldoze' all your FIS Modules before you build the Emergency Generators.