if i get a new phone but same phone number, will i get the sent but undelivered messages people have sent me?
As long as it hasn't been 30 days since you lost your phone, you will receive all pending messages. But if it has been more then 30 days, you will not receive those messages, as WhatsApp deletes them from their servers.

From WhatsApp's FAQ:


  ...undelivered messages are deleted from our servers after 30 days.


On a side note, if it has been over two months since you lost the phone, then it's as if you're a new user on WhatsApp, and you will be removed from all of your WhatsApp groups.