With 7.3.5 you can save up to 10 levels at 20 with starter edition.
Asked an Employee:
Basically, you can "save" up to 10 levels when you have Starter restrictions.When those restrictions are gone (That is, when you purchase a WoW account/gametime) you will get those levels at once :)
Keep in mind you CAN'T SEE how much XP you have banked and any extra levels past 10 will be irredemably lost, so if possible it's better that you don't level up at all when you have these restrictions! This is meant as a bit of a "safety net" so you don't lose your well-deserved XP if you continue playing for a bit after you reached the max Starter level :)
And now, there is a Problem that you cant see your gained exp though questing or Grinding tried some experience addons but they are not working, someone know a simple method to create a textbox like X to level where it shows gained levels and earned xp ? My gametime is over now and i dont want to reactivate until the next content unlock so i can twink if i want and save up to 10 levels with my low level chars.