Missiles of the R-36M/SS-18 family have never been deployed with more than ten warheads. But given their large throw-weight (8.8 tonnes is the official START number), it is easy to see that they can be made to carry much more than that. That was something that the United States worried quite a bit in the 1970s. And, as it turns out, rightly so.
Among the projects that the Soviet Union considered in the mid-1970s was that of a 15A17 missile – a follow-on to the R-36MUTTH (15A18). The missile would have had even larger throw-weight – 9.5 tonnes – and would be able to carry quite a few warheads. Five different versions of the missile were considered. Three of these would carry regular warheads – 38 (yes, it’s thirty eight!) with the yield of 250kt each, 24 500kt warheads, or 15 to 17 1Mt ones. Two modifications were supposed to carry guided warheads (“upravlyaemaya golovnaya chast”) – 28x250kt or 19x500kt.
Similar plans existed for another missile – UR-100NUTTH/SS-19 (15A35). Its 15A31 version, had it been developed, would have carried one of the following: 18x250kt, 11x500kt, 7-8x1Mt regular warheads or 12x250kt or 9x500kt guided. To be able to do that, the throw-weight of the missile had to be increased to 4.6 tonnes (the value for UR-100NUTTH is 4.35 tonnes).
Fortunately, none of this has ever materialized. The SALT II Treaty, signed in 1979, prohibited increasing the number of warheads ICBMs can carry. I’m sure other considerations played a role as well – a simple “strategic stability” estimate would show that putting that many warheads on vulnerable silo-based missiles is not exactly a smart idea – but I certainly would not underestimate the role of arms control.
So, eventually, things stayed where they were – R-36MUTTH was deployed with ten 500kt warheads, its follow-on, R-36M2 (15A18M) – with ten 800kt warheads (single-warhead versions with either 8.3Mt or 20Mt warhead also existed at some point). UR-100NUTTH is deployed with six warheads, 0.4Mt each.