Hmm, how many half-bloods are there?

From Wikipedia:

Half-bloods are the most common wizard blood, far outnumbering pure-bloods and Muggle-borns. Rowling has stated that of the Hogwarts annual intake, 50% are half-bloods.

Though yes, 'half-blood' here refers to any wizard that isn't muggleborn or pure-blood. So a great deal of them might just be descendents of muggleborns.