I am a founder of an enterprise software startup and about to file provisional patent(s) on which the software is based. I realize the advantages of bypassing the provisional application, but the cost is the limiting factor. The invention is likely going to be multiple patents. From what I read, a provisional app may refer to another provisional app by reference, but that's a hairy route because if the referred application never examined, the reference is invalid. People have suggested just copying the relevant parts of the reference into the dependent application verbatim. I am also reading that multiple regular applications may be based on and claim priority date of a single provisional app. Should I then describe the entire invention in a single provisional app and not worry for now how many regular apps it will end up being. The reason I hesitate is that a patent application cannot claim multiple inventions, which, in effect, is what this single provisional app will be doing.
I would put the whole system into one provisional. (Oh - a provisional is never "published" per se but it is made publicly available when any application or patent that gets priority from it is published.) A non-provisional filing possibility is filing simultaneous applications for each major subsystem (if relevant) They would all claim the benefit of the provisional. They could have identical content other than the title, summary and the claims. Those sections would be targeted to the specific subsystem. While a patent can cover only one invention, that requirement doesn't come up very often in practice. More likely, is the examiner issuing a restriction requirement stating that the claims cover things that are different enough that it would be a substantial burden to search and examine them separately. At that point you pick one and either acquiesce to the restriction or try to fight it to have more breadth in the resulting patent. Whatever doesn't get examined in a specific filing can be addressed by another filing. That can be done immediately or anytime later as long as there is a "live" application that chains back to the present application or to any application with sufficient support for the claims.
If there are different inventors of the various inventions there is a possibly of some applications being used against each other if not filed on the same day.