In general - there is no limit, but you need to be aware of a few things.
- This is not a normal shell, we use libguestfs to execute the commands.
- Executing may take more time than executing in normal shell you're used to. If you run BG in a VM - it will be even slower.
- You have a memory limit by default set to 300 MB, but it can be extended by setting LIBGUESTFS_MEMSIZE env variable.
I advice to execute only required things. If you have some very heavy tasks, consider doing it a first boot or preapre the tasks to be executed faster. Another advice: don't let grow the post section too much - better use the files section, inject scripts and execute them in post section.