Yeah, my Napoli pants are extremely slim. For the La Spalla and Lazio models, I had to have them taken in at the waist and seat. The Napoli were just right at the waist, but the thighs are so tight that I feel like I will rip them open just by sitting down. That, along with the jacket being too roomy, make the Napoli the most oddly-proportioned suit (at least in my mind).
^ageed, though if the OP is wearing these shirts every day, he will need more than 7 to start with. I'd say you'll need 14 so that you have some to wear while the others get cleaned.
Also, I'd skip the extra pants right now. As others have said, there is a chance that, in 6 months or so, as you've gone through the process, you won't like anything you bought at the beginning. I'd minimize the investment and stick with 1 pair of pants per suit.