Rockwell has an embedded HMI that comes inside their panels (formerly called PanelView) and a Windows based HMI product called RSView. The tremendous number of Rockwell panels installed makes PanelView one of the most popular HMI's around. I believe they sell RSView for a very low cost, hoping that it will pull through additional hardware sales. But to answer your question, the installed base of any HMI product is going to be very difficult to obtain. The only place you can even come close to getting verifiable numbers is from a company like ARC (http://www.arcweb.com/market-studies/pages/human-machine-interface-software.aspx) which sells the stats.