There is no "group" command in an assembly but as stated you could "Demote" those 2 shelves into their own subassembly. (Demote is the same as just creating a new assembly and placing those parts into it and giving it a new filename)..
Then you could go into the document settings for that and set its bom structure to "Phantom" which would cause those parts to be pushed up into the upper level BOM/Parts list so instead of seeing the subassembly in your upper level parts list you would see exactly how many shelves you placed into the assembly.
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Inventor 2023 - Dell Precision 5570
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