Rule 106.12

One card (Drain Power) puts all mana from one player’s mana pool into another player’s mana pool. (Note that these may be the same player.) This empties the former player’s mana pool and causes the mana emptied this way to be put into the latter player’s mana pool. Which permanents, spells, and/or abilities produced that mana are unchanged, as are any restrictions or additional effects associated with any of that mana.

Find this number in the current Rules