its a number
If you mean 999, that's the UK version of dialing 911. Otherwise, the string has no operators and could just as easily be a 4-digit lottery number.
If you add it all up, the answer is 72.
A sequence of numbers with any symbols between them are generally taken to represent decimal representations. In this case, it would be 999999 or 1 less than a million.
99 plus (99/99) = 100