How much Damage does Mother Brain's Hyper Beam do?

If you have the Varia Suit, it deals 300 damage (that's three full energy tanks). If you do not have the Varia Suit, it deals 600 damage (six full energy tanks). In a minimalist run (assuming you're actually going to beat Mother Brain and not skip her entirely using some extensive glitching), you would need at least three energy tanks. There is a FAQ by one Kejardon here on GameFAQs called "15% Walkthrough" which tells you exactly what you need and how to beat the game with the bare minimum.