Why does htop show lots of apache2 processes by ps aux doesn't?

HTop is showing all the processes and their threads whereas ps aux only shows the processes. Press F5 in htop to create a hierarchal view of the processes and their threads. It will make more sense.

sweimann's answer is correct.

Additionally you can make ps(1) behave in the same way by using the H flag which will display threads as if they were processes.

Apparently the same key will also toggle off such behaviour in htop.

I don't know htop, but maybe it displays threads as separate processes and you have configured apache with a multithreaded worker?

In htop, you can disable this behavior by pressing F2, then go to "Display options", then mark "Hide userland threads". Optionally you may find "Hide kernel threads" useful too.