Yes, you can run latest iOS. But it is dog slow. Try to use it with at least 3 apps. For a typical user with > 10 apps it is useless.

Not according to reviews. According to reviews, IOS 12 is very performant for 5s. I know it works well on the 6s series.

The number of applications installed doesn't matter. iOS will purge apps if they are running in the background and take up too much CPU or memory. You're not going to have > 10 apps running in the background. More than likely, you won't have any third party apps using CPU time in the background besides playing music or VOIP.

EDIT:

Just realized that you may be referring to macOS. In that case, that would be RAM and/or disk dependent (SSD vs spinning hard drives).

I ran it on an iPhone 6 and it was fine. Your comment about "at least 3 apps" makes very little sense to me; iOS is notoriously aggressive with memory management, and apps must be designed to expect being killed at any time without warning, with only a few specific background tasks allowed to run (and generally for limited times with explicitly defined purposes). I'm sure my phone had upwards of a hundred apps on it, but generally speaking, only one or two were running at any given time because that's true of any iOS device.