If we are being exactly correct, it should always be "there ARE two of something." However, in spoken English, you may hear "There's two ...." You will always hear it as a contraction - almost never as "there is two...". It is an error, but it ends up being said a lot in everyday spoken English. If you say it, I doubt anyone would correct you. It's better to use the correct form, though. "There's two guys outside waiting for you." [Correct: There are two guys...] "There's four cars parked in the driveway." [Correct: There are four cars...]

It's "there are two numbers" in this case. It can be a little confusing, such as "there is a pair of candles on the table" and, of course, sometimes people are lazy and say the wrong thing. :-)

When to use "is" or "are" depends on whether the subject is singular or plural. E.g. There IS a number... There ARE two numbers... In one of the other answers given, this example was given: There IS a pair of candles on the table. The reason this sentence uses "is" is because it is the pair that is singular. I.e. You only have ONE pair. So: There IS a pair of candles... There ARE two candles... In response to one of the other answers offered regarding contractions, I don't recall hearing people use the wrong contraction very often. It would sound strange to me. In speech, I tend to say, for example: (There is a car outside). There's a car outside. (There are two cars outside). There're two cars outside. If someone were to say to me, "There's two cars outside," it would sound odd to me.