If I attack with Venonat's Leech Life and do 10 damage, how many damage counters do I remove from Venonat?
You remove one damage counter, which equals 10 damage.
Source: WotC Chat (1999-09-30)

Relevant cards

Note: All applicable cards may not be listed below.
BW1_45
BW1_46
HGSS4_81