How many oz in a mug have?

Mug sizes vary, as well as the term "mug". 

By definition the term mug generally refers to a sturdy built type of cup often used for drinking hot beverages such as coffee, tea, or hot chocolate. 

Mugs by definition have handles and often hold more liquid than other types of cups.Usually a mug holds 12 ounces of liquid, a double tea cup. In formal settings a mug is usually not used for serving hot beverages, a teacup or a coffee cup is generally preferred.

1oz=31.1035g 
1oz=29.571 ml
Generally speaking, one mug for 300ml is 11 oz, the size of the mug is 8cm x 9.5cm.