6 Spice Jars (Mrs. Dash size, approx 2" dia.) will fit across each shelf on a standard sized (12" wide) spice rack, so a 3 shelf spice rack will hold 18 jars, 4 shelf rack 24 jars.

7 Spice Jars per Standard Sized (12" wide) Shelf

Jars per shelf on custom sized spice racks:
12" wide Spice Rack fits 6 jars per shelf
14" wide Spice Rack fits 7 jars per shelf
16" wide Spice Rack fits 8 jars per shelf
18" wide Spice Rack fits 9 jars per shelf
20" wide Spice Rack fits 10 jars per shelf
22" wide Spice Rack fits 11 jars per shelf
24" wide Spice Rack fits 12 jars per shelf