There are no limits to how many times you can feed as a vampire. Feeding on an NPC won't lower the health of or kill an NPC.  Also, feeding always returns you to stage one vampirism, so there's no need feed multiple times at one time.

More info at the [UESP wiki's "Vampirism" article][1]


  [1]: http://www.uesp.net/wiki/Skyrim:Vampirism