That’s a good question, I have had people start with as few 2-3 units and all the way up to 200-300. Several factors need to be taken into consideration. How large is the population of your service area, what is the construction activity environment like(Is it growing or is it slowing down) what is the potential for doing special events( carnivals,parades,fairs). I would say research these items, visit some contractors to do some pre-selling to get a feel on how many units you can put out on rent in the 1st month. Once you have that number, I would order that many plus 15-20 units extra for future growth and also to have on hand for special events. Once you start I would come up a inventory number of units that you have available for rent that once you fall below that number it means it is time order another load of units. The last thing you want to happen is to get down to 1-2 units and all of sudden you get that call for 10 units to be delivered ASAP.
I started with 3. By the end of the first year I had over 80. We were lucky to be able to purchase units as we needed.
On a related note, I would highly encourage you to read the article on this site about branding. It’s very good. As a startup you will want to make yourself memorable. Having a great logo and everything look the same will help that. Great customer service will help too!