It really depends on how complex you DVD menu system is going to be. If you have a lot of chapter points you don't need to try to squeeze them on on the same menu page. Just personal preference, I think anything beyond six is pushing it.
Spain (Main Page) contains 2 submenus: video & photo
If you choose "video", it enters the "video - index" page. Yet there are 13 video clips to play, and one index page only contains 7 thumbnails, so I need another page ("video" page 2) for video clips no.8 ~ 13.
Submenus are "under" the menu; page 2 "follows" page 1.