There's probably a declared number where this matters, but the current number of warheads is high enough that's there's no need to make more. 3,000 is plenty to retaliate against an opponent with 30,000. More doesn't provide a benefit.
Nuclear disarmament, as practiced by the US and Russia is a negotiation to reduce the number of warheads in a coordinated fashion so that it's possible to convince warmongers on both sides that it's reasonable. The benefits are primary a reduction in cost to maintain and secure the warheads and a significant reduction in the risk of accidents related to the warheads. Mutual destruction is still assured --- you'd need a lot fewer warheads for that and involvement of other nuclear states; but then your question of production capacity would be more interesting.