Herd Immunity (https://en.wikipedia.org/wiki/Herd_immunity) means that everyone doesn't need to get it. The threshold for COVID-19 is estimated to be between 29-75%.

On a different note in scientific paper what is considered too large an error margin to consider it a value to talk about. If the above is true that makes it what 52%+- 23

Assume that everyone who has it spreads it to 2 people on average before dying or recovering. If one of the 2 people you would have spread it to is immune, you will only spread it to 1 person instead, and the total number of infected will not rise.

Huh? If you spread it to 1 person, then the total number of people who have been infected will rise by 1.

I mean the total number of people sick at any given time, not the total number of people who have ever been infected.

The parent poster asked "Why would we only expect half the world to get it?". It sounds like your answer to that is "no no, almost everybody will get it, they'll just get it at different times".

If the effective transmission rate is 1, yes, more people will get it, and then the effective rate will go below 1, and you enter exponential decay.