Hi
I want to make tubes with a length of 550mm using uni-directional prepreg, If the width of the prepreg sheet is 300mm, how much overlapping can I have while stacking the layers around the mandrel ?

You would make a tube with the ud running length ways so your length is immaterial you would cut material the same length as your mandrel
How many times your ud would wrap around your tube depends it's diameter an how thick you want it
Get to the end of your material just butt join another section on