It is okay to clean multiple items in one ultrasonic cleaning session. However, the items in the tank should not be too much. The tank should not be filled with items. That is because the items will be stacked. When items are stacked inside the tank, improper cleaning happens. Some surfaces are not cleaned because the ultrasonic cleaning solution cannot get in contact with them. The contaminants on those surfaces are not removed.
To be sure that proper cleaning will happen, space the items properly so that all surfaces of all items are accessible to the ultrasonic cleaning solution.