Yes, it's one shot. Some folks use the higher dilutions for the economy but most are interested in the acutance effects evident in the higher dilutions. The 1:25 and 1:50 were the original specs from Agfa, some films do better with the higher concentrations, some need longer times to get more even development. Rodinal is one of those highly personal developers, you can choose a usage that fits your way of working, from a fast working 1:25 on through stand developing at 1:200 for an hour-development limited by developer exhaustion for contrast control.