Do I need to install a gable vent on every gable?

I’m replacing the siding on my house this week and noticed only some gables have vents, and some attic spaces without roof vents are really hot. What is best here? A gable vent on every gable to promote ventilation? Install a combo of gable and roof vents? I know the soffits should be vented, but they aren’t nor will they be on this project, if ever.