300K orders a month is approximately 10K orders a day, or 400 transactions per hour. Of course, you'd get more at peak times, but that still seems like it could be handled on a single server. You might want to have two servers for increased availability (e.g., servers in different regions, in case there's an outage), but having 10-20 servers means that each of your servers will be idle almost 100% of the time.
If your business has strange order patterns, e.g., 90% of the orders arrive the week before Christmas, then the analysis would be different.