No, one is actually enough in each country they deal with any many regions have trans-national carriers so it is a case of registering with the national communications regulator and coming to agreements with some of the bigger players.
If you take the Australian market, the vast majority of MVNOs have agreements with just a single MNO and an IoT service (or over the top or content service provider service) is covered by different legislation than that of Carriers and CSPs, if at all.
