What is the maximun weight a longboard can hold?

This question is very broad. This depends on the construction, length, width, and shape of the specific longboard. A bamboo or maple longboard will not be as strong as a longboard that is reinforced with carbon fiber or fiberglass layers. A shorter longboard will be less prone to breaking because it will be stiffer. Just research the materials, length, and shape to see approximately how strong a specific board is.