It depends on the size, scope and time duration of the project. In light of our experience as a UI UX design agency, we do not confuse our projects with overloaded workforces. More than one designer for a single project sometimes complicates the design process. In this case, we assign a lead UX designer over multiple designers to avoid unnecessary issues.
Unless the project consists of a complex interface, architecture and tight deadline, we normally assign a single UX designer for a single project. Considering the depth of the hurdle and the necessity, this number will rise.