Lennitard, you may not want to get into creating custom pushpins; it is potentially a lot of work.
The closest I can get to your objective is to have a set of pushpins that show only the number in a pushpin flag, or "balloon." Those balloons come as a default size and the minimum size is two lines, one for the name and one for the address. However, you can get your item (or location) number on to the top line and leave the second line blank which is about as clean as I can make it.
To do that, I create a spreadsheet with the first column containing the number sequence. Like Ken's spreadsheet, the other columns contain address data or whatever you like. When I get into the Data Import Wizard, I make sure the first column, the number sequence column, has "Name" as the data type. Then, when I have finished the Data Import Wizard, I select the new pushpin set and go to Properties. There, I select the fourth tab "Balloon" and make sure all boxes are UNCHECKED. Then, select "Show all pushpin information" from the right-click drop-down menu for that new set; that will give you the result I describe above.