To ensure that our deterrence remains effective in the face of US plans to 'neutralize Pakistani nukes' and developments in ABM technology.
The more nukes, the harder to 'neutralize' all of them, the harder to 'destabilize Pakistan' because of the threat of nuclear material in a destabilized state falling into the wrong hands, and the harder for the world to support US attempts to attack Pakistan given the fallout from a larger and more powerful Pakistani arsenal being used.
Just because Pakistan ends up having over a thousand nukes does not make it any more likely that Pakistan would use them unprovoked.
Whether it is one nuke or one thousand, Pakistan's policy of only resorting to them in case of a significant threat to its existence or national security remains in place.