An intensified spell increases the maximum number of damage dice by 5 levels. You must actually have sufficient caster levels to surpass the maximum in order to benefit from this feat. No other variables of the spell are affected, and spells that inflict damage that is not modified by caster level are not affected by this feat.
For every two caster levels beyond 1st, you gain an additional missile - two at 3rd level, three at 5th, four at 7th, and the maximum of five missiles at 9th level or higher.
First of all, would intensify work correctly on magic missile? Second, how exactly would its effect occur? Would it result in 2 extra missiles by allowing the increase of caster level, or would it be 5 extra missiles as each missile does a single damage die? Would it work in a way that I have not listed?