The number mounted matters cause it determines how missiles can be used. With multiple warheads, ICBMs could be used for first strike. First strike won’t work,

The number mounted matters cause it determines how missiles can be used. With multiple warheads, ICBMs could be used for first strike. First strike won’t work, but opponent still needs to worry about it. With one warhead, ICBMs are only useful for retaliation strike. Also, the US is still following New START treaty after Russsia pulled out.