Well.... no not really. You can have as many administrators as you like. There's no restriction imposed. As Paula points out, though, it's probably not the best idea for day to day operations.
Most of my sites have two administrators to provide some cover, but these are not 'normal' user accounts (username 'admin' is an Administrator, username 'howard' is a Manager if you see what I mean).
The first admin is the 'primary administrator' and cannot be deleted. They have no more rights than any other administrator who may be assigned later. In fact, the administrator account works by simply overriding all capability checks - the answer is always 'yes no can'. Which is why it is so powerful/dangerous.