Only one word:
^[a-z,0-9]*$
at least two words:
^[a-z,0-9]* [a-z,0-9]* (add a dollar sign at the end for exactly two words)
at least three words:
^[a-z,0-9]* [a-z,0-9]* [a-z,0-9]* (add a dollar sign at the end for exactly three words)
etc.

These regex will exclude search phrase containing non-alphanumerical characters, so if you happen to have search phrases containing those (c++, c#…), you can insert them the characters between the brackets, comma separated.