Not exactly. My 50K center (U21 - domestic), got me a boost of about 15K merchandise.
You get the additional founds only when your NT players are playing in official matches (when the season was over, the merchandise dropped). If you do actually get anything in scrimmages it's pretty insignificant.
If you designate 30 players in NT would all make the same merchandise compared to just 8 players?
No, the total merchandise would be different. It depends on salaries, nationality, and if they actually play in games or not. I'd say the 30 NT players would get more merchandise revenue (except maybe if those 8 players are 500K+ behemoths, compared to 30 low salary guys).
Generally NT players rank more then the ones from U21.