More symbols are available from extra packages. Infinity is not a number. As it is virtually impossible to list all the symbols ever used in mathematics only those symbols which occur often in mathematics or mathematics education are included.
The symbol is encoded in Unicode at U221E INFINITY and in LaTeX as infty.
The Not So Short Introduction to LaTeX is also an excellent resource. There are no approved revisions of this page so it may not have been reviewed. Garciaj Dec 12 13 at 1927. Symbol L a T e X Comment.