This is a good question but not a simple one to answer purely in theory. A lot of the answer hinges on how isolated the threads are in the Apache/PHP architecture. While there may be separate threads, that doesn't mean there is no contention between them. For example, they may lock on shared mutexes or semaphores. In that case having 5 containers executing 1 request each maybe able to handle more concurrency than 5 threads in one application instance.
As far as whether there will be more thread context switching. That isn't necessarily the case given the same number of requests. However, if there is one or more non-request thread per instance, you would have 5 times the number of these overhead threads.
I suggest trying various configurations under load and get metrics for better understanding of the costs and benefits of each.
There are other benefits to isolation though that may outweigh such costs if they exist. One major benefit is that if you have one container crash from a bad request, you will not lose the other 4 and a lot of tooling allows to automated restoration of downed containers. Also if you are subject to any of the many vulnerabilities that are regularly found in PHP packages, an attacker will have to escape from the container in order to gain access to or control over the host/VM.