It has nothing to do with ERPNext user limits. You can create as many users in ERPNext as you want. DO pricing is based on vCPU, RAM, SSD and data transfer. Plus you get a static IP for each droplet. You can start with the 1 GB memory and upgrade when you feel it is slowing down.
Usually, the $10 droplet with 2 GB RAM works very well. In addition you can swap your SSD if you need more RAM power. There is no magical formula here, it all depends on your use case.
Hers a link of ERPNext installation I shared for another user who was facing an issue. This works for us perfectly, never had an issue (touchwood!!)
Hope this helps.