Can a "normal" user see all processes with htop? Can I see all processes with "htop" or does it make sense to use "sudo htop" ? 

I use htop in the terminal (installed via brew / homebrew). My useraccount on the macbook is a default user no special rights restrictions. OS-Version Mojave.

https://formulae.brew.sh/formula/htop
Yes, you can see all processes with htop as the default user.

Note that if you for example use virtualization to run multiple systems on one, htop will only show you the virtualization program as a process - not the processes running within that virtual machine.