We have a big one AND WE CAN DELIVER IT. If we miss by a mile or two, ...meh.
In theory there is no limit on how many stages thus yield one can have in the Ulm-Teller design. There were stories the Ruskies were interested in building a doomsday ship. The ship would be one giant thermonuclear weapon and if it stopped receiving some signal it would go off.
We have a big one AND WE CAN DELIVER IT.
Gort, forget klatu barada nikto. Go to plan B.