How to store the result of a command expression in a variable using bat scripts?

I have the command below to count all directories that that follow the pattern 20?????? :

'dir /b "20??????" | find /c "2"'

For example, if I have the following directories, the command would return 6:

20090901
20090902
20090903
20090904
20090905
20090906

How can I store the result of this command (6 in the forementioned example) in a variable?