Currently, the maximum number of spacefarer modules that can be built on a given map is 16:
However, it is possible to exceed that number by queueing up multiple spacefarer modules while below 16 and exceed the maximum of 16 allowed:
1) build 16 spacefarer modules, notice the maximum is reached and it is impossible to build more
2) deconstruct 1 spacefarer module
3) queue up multiple additional spacefarer modules (in the example above, I queue up 5)
4) let duplicants build the spacefarer modules
Congrats! You now have more than the limit of 16 spacefarer modules!