I have worked in a company with a monolith and about ten teams working on it. This is what helped:

- Merging was automated (a robot tried to run tests against fresh master and merge only if green).

- Deploy was fully automated and limited to working hours.

- We added tests for problematic parts. For example static analysis for database migrations to prevent only safe actions in an automated fashion.

However, if something goes wrong in some component, you have to revert and stop the deploys for everyone which sucks. I'd say around 8 - 10 deploys per day, it makes sense to start splitting the components or at least not adding new teams to the same monolith.