How many teaspoon of milk is needed to melt 50g of chocolate?
You do not need any milk to melt chocolate. The goal with melting chocolate is to keep it within a range of heat so that it doesn't burn or separate.

A bain-marie is a technique where you place a heat-proof bowl over a pot of simmering water to heat the contents of the bowl slowly.  If you're melting chocolate for dipping bananas, I would recommend using the microwave technique (mentioned in the comments).  It's a lot easier. Just do short bursts (10-15 seconds) and stir in between each round.

Aside: If you want to go all out, you can read about tempering chocolate. Tempering chocolate will help the chocolate set with a nice shine.  It will also set at room temperature.  I avoid all this and throw the bananas in the freezer for a few minutes after dipping them in chocolate. I hope this helps!