Get data! Let your customers tell you through the data what you need to build. I would start by building the one browser you know for sure you're going to support. Roll it out and review your analytics to see what browsers they are using. If you have a browser that you don't support that is providing a decent % of your traffic you can prioritize that as your next browser to be sure you're compatible with. Much cheaper and quicker to let the data determine your development path than spend too much time on assumptions. At the end of the day it ultimately is just an assumption until you have data from your audience (not competitors or similar products and services).