I think it depends on your opponent. If your opponent is a technologically and militarily capable nation which is capable of identifying (via HUMINT, SIGINT, IMINT or whatever means) your missile launch sites and or capable of tracking your SSBNs then the more nukes there are the more deterrence projected.
It is right that spreading your nukes into SSBNs, land based silos, mobile launch vehicles, bombers, rail based systems etc will help in increasing the deterrence, but capable enemy can locate and/or track these launch systems and therefore in this case there more there are, the better the deterrence.
As of 2011 the Russian Federation possesses just under 1600 strategic warheads and the USA just under 1800 strategic warheads in active duty.