I need the several more dolls' - we never use 'the several' The proper alternative would be: 'I need several more dolls'. 'I need several more of the dolls' - this is fine. The difference is that in the second sentence (of the dolls) you are talking about a specific set of dolls we already know. In the first sentence (more dolls) you are asking for dolls in general.

"I need the several more dolls" is not proper English. You should say "I need several more of the dolls."