General:
* Understaffed
* Short-staffed
* Undermanned (more formal and often used for male-dominated professions)
* Under-resourced
* Staffing shortage
* Personnel deficiency
* Labor shortage
More specific:
* Depleted (implies a significant loss of personnel)
* Under-equipped (lacks the necessary personnel to perform tasks)
* Overworked (existing staff is spread too thin)
* Unstaffed (completely lacking staff)
* Skeleton crew (minimal staff is present)
* Under-performing (lack of staff leads to lower productivity)
More informal:
* Lean (minimal staffing, usually implying efficiency)
* Bare bones (minimal staffing, often implying a struggle)
* Strapped (understaffed and struggling financially)
* In a bind (understaffed and facing a difficult situation)
The best word to use will depend on the specific context and the level of formality required.