The answer to this topic is mostly about the definition of "number", as Username said in some other thread. For those that didn't get it, let me elaborate set theory a little bit.
Start with zero, the empty set: 0 = {}. Next, the number one is the set that contains the empty set: 1 = {0}. The number two is the set that contains the empty set and the set with one member: 2 = {0,1}. Three is the set containing the previous sets: 3 = {0,1,2}.
You should quickly be able to see that the counting numbers are sets that contain all the previous sequential sets. If we then say infinity is the set containing all these counting numbers, ordered sequentially, then one can conclude that infinity is indeed a number.
One can argue that such a set cannot be constructed and thus is not real, but a look back through the history of mathematics should persuade such an argument. It was little more than 100 years ago that most mathematicians considered pi to NOT be a number since it cannot be represented as a fraction of counting numbers. Such a number as pi and e and the square root of 2 were given the name irrational as a form of derision, but the name lost its intended purpose as they become useful in everyday life. The same can be said of imaginary numbers, surd (as in absurd), and zero, too. Even though they cannot be constructed, they are useful to us as numbers.
Infinity exists in mathematics and is used everyday in every facet of your lives, even if you don't know it. Based on the only consistent and fundamental definition of a "number" (as shown by Georg Cantor), infinity is a number. Even though in conventional maths it is used as a trend or a limit, not a number, the rules regarding its behavior are consistent with set theory.