I think 10.3.2 will be the last iOS 10 beta and update.
At the pace that security updates are required, and knowing that the next iOS update is likely this September, I seriously doubt that 10.3.2 will be the last. There may not be a 10.4, but I expect that Apple will need to continue security patches going forward.
Just as a "for example", 9.3.1 came out almost exactly a year ago, and then there was a 9.3.2 in May, 9.3.3 in July, 9.3.4 and 9.3.5 in early and late August.