What is smallet number
There is no such thing as the smallest number, because no matter what number somebody suggests, I can always subtract 1 from it to make another number that is yet smaller. Take note that we're talking about numbers.
There's a concept called infinity, which means everything in the universe, so since it's already everything there's no way to make it larger. It might be a stretch, but IF there could be a negative infinity – which I won't even try to explain – one might argue that negative infinity is the inverse or opposite of infinity. One might, but I'm not that one. But always remember that infinity is not a number, and we can not use it in calculations the same way we use a number.
What we can do is use infinity as a concept. It's legitimate to talk about something like the limit of y as x approaches infinity. That's an asymptote and we see those all the time.