paulh16 - You mentioned seeing this in the Network Admin's Dashboard. In which widget do you see the message? Are any numbers in the messages there missing?
For example, in the case of one of my sites, the error appears in the "Right Now" widget and there is a missing number in the message "You have 1 site and users." (in my case, this network has over 17000 users).
The missing number can give us an idea which call to number_format is causing problems.
This error seems to happen a lot. I've seen it on multiple sites, including those of some of my clients. In many cases the cause can be traced to a plugin. But there are a lot of unsolved cases.
There is a related bug here:
It's not clear to me if this is related to the issue we're seeing or not, though.
I'm not saying I have a solution, but wanted to post what I think is useful related info.