How much muscle should our bodies have?

You always hear about how much fat is too much fat, but I would like to know how much muscle is too little.
You always hear that as we get older our muscles start to shrink and how inactivity will lead to less muscles and bones. However I have no idea as to what is considered a low level. There are ways to measure for BMI but I haven’t found a way to measure for muscles on the internet. I’ve been working out and toning up but I’m trying to get to optimal level for health and not have to bulk up like a guy to then say I have enough muscles now.