how can we restrict users to selecting only ONE specified issue type at issue creation?

[Pedro Cora]

Not possible by default in JIRA. But can be done with Script Runner. See this answer for details. 

[Joe Pitt]

There isn't a way out of the box. You can search the forum for some hacks, but what happens is all the issues are displayed and when they select the others they get an error. Frustrating for them and makes you look unprofessional.

[Jen-Qology]

Our goal is to have end-users only select one issue type, and any other selected, should prevent them from moving forward to create ticket