Peoples is the plural of people when people means a nation or ethnic group, or the like. For example, there is a book called Africa, its peoples and their cultural history. This title does not indicate a history of individual Africans, but of African tribes and nations.
People is the usual plural of person, meaning individual human, although one also sees persons. When speaking about a number of individuals, peoples is never correct.
In the phrase "capacity of 150 X" it is clear that you are referring to 150 individuals, so peoples is incorrect. The phrase should be
... capacity of 150 people.
However, "persons" is more likely to appear in legislative and regulatory texts, so it would not be unusual to see an official notice restricting a room's occupancy to
... no more than 150 persons.