3,500 nuclear weapons could still get the job done I think. Imagine the consequences of two major cities of your opponents getting destroyed. Lets say Moscow and St. Petersburg and Shanghai and Beijing. Drop a 300KT weapon in the center of each of them and those countries would be reeling for quite a long time.
assuming all are able to lauch. Many could be destroyed in a first strike, some fail to lauch (i.e. see challenger explosion. rockets are not 100%), and the enemy doesn’t develop an effective anti-missile defense. The idea behind thousands of kill vehicles is that some would get through. Reduce the number and you increase the risk someone might try something.
Heck, 350 operationally deployed would probably be more than enough.
The principles of MAD, counter force, counter value, etc still apply despite Freeper droolers. ;-)