Show the total no. of component items on the Item card which is a bill of material

If you want the simple info, create a flowfield field on Item table and in the CalcFormula place the:
Count("BOM Component" WHERE (Type=CONST(Item),No.=FIELD(No.)))

The complicated info is counting the unique items that an item appears in their BOM component tree.
That would require some coding to be involved.