Here are some options depending on the context:
Formal:
* Panel of umpires
* Umpire crew
* Umpiring staff
Informal:
* Group of umpires
* Team of umpires
* Squad of umpires
You can also use more descriptive phrases like:
* The umpires who officiated the game
* The men (or women) in blue (referring to baseball umpires)
* The officials
Ultimately, the best collective noun will depend on the specific situation and the desired tone.