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.
Maybe with India or Pakistan's arsenals, but with the Russian, Chinese and US arsenals it is a different story. A first strike would be very risky and would most likely not disarm your opponent. 3,000 weapons across the triad are hard to keep track of and even harder to kill all at once.
A more likely scenario would be attacking a limited number of military targets to get the point across or even go as far as city trading.
Exactly, if we were to launch a massive nuke strike, I would bet that the first wave would contain a large number of fakes to deplete the anti-missile defense. Knowing that, you need twice as many missiles to overwhelm the defenses.