When I am printing multiple items on the same bed, how can I enable support only for one item?
You have two options: Either select the objects tab in the left pane of Bambu Studio to select a specific support setting for an individual object, or select one of the manual support types and use the support brush to paint all the spots you need support.
I’m not seeing the “Enable Supports” line item when I click on a part. Under “Frequent” I see Layer Height and Sparse Infill Density only. Supports are enabled globally.

