Modern H-Bombs are based on the Teller-Ulam design.
With this design, one can attach as many stages as required to up the yield.
The only practical limit is the method of delivery. Quite a ingenious design.
The largest bomb ever detonated was "Czar Bomba" in Russia with a yield of 53 MT using a modified 3-stage Teller-Ulam design.
The original design goal was 100 MT, but was reduced due to fear of fallout.
Even then, the power of 53 MT was incredible, even by nuclear standards.
The ground impulse was measured at 7.1 Richter and this was an air burst which dissipated most of the energy into the air.
The most powerful US nuclear weapon was the MK 41 at 25 MT.
"Czar Bomba" was an impractical political statement, while the Mk 41 was a deliverable weapon system.
It was discovered early on that for surface targets, the use of multiple, more easily deliverable warheads surrounding a target, was more efficient than one large warhead.
The overall amount of area using smaller warheads was greater than one large one.
This is why, for instance, the single 1.2 MT warhead used on the Polaris A1 and A2, was replaced by three 200 KT warheads on the A3 that landed as a triangular group around a central point.
The three warheads together had greater destructive range than the one warhead.
After this, came the development of MIRV designs, where several groups of warheads could be carried and aimed by one missile was introduced in the early '70s.
An example of this is the photo below showing 8 warheads from a single missile.
After this, the MARV was developed that allowed each warhead the ability to steer and correct for drift and wind currents.