The number of peers is mainly limited by memory usage. A commonly used value us around 20.
Regarding your main question: Running servers does require a little determination to set things up properly. It is not all done by pushing money in Bezos’ pocket expect AWS to shield you from work 
Using systemd is one way to keep your node running if it crashes or your system is being rebooted for whatever reason. For this you would not need tmux at all - although some people (including me) think it might be helpful to be able to connect to live view. Without systemd, you could use tmux to keep it running without setting up a systemd service - just detach from the session and logout. Although being easier to setup (cardano-node is quite stable) this would not be my preferred way of running a service.
I suggest you have a look at the tons of well writen tutorials and explanations around. For example here.