Joltik

If Ariados' "Leech Life" attack does 30 damage but the opponent's Active Pokemon has only 10 HP remaining, how many damage counters will be removed from Ariados?

Leech Life does 30 damage, which is not affected by the fact that the KO only required 10 damage. Therefore, Ariados will remove 3 damage counters, or all if there are less than 3.

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.