number of Model, question

[eluser]imamiscool[/eluser]
In CI, if I want to make Model class,

should I make one Model for every table on my database
or
I make one model for each controller ( ie. each page have one corresponding model )

example:

contoller:
-Home
-User
-Order
-Register

database<my_site_db>:
-users (user_id, user_name)
-orders (order_id, stuff_id, order_total)
-users_orders (user_id, order_id)


should I make models:
this: UserModel, OrderModel, UserOrderModel
or
this one: HomeModel, UserModel, OrderModel, RegisterModel



I am stuck on creating function on Model which is JOIN several tables
Where should I put thefunction, ( which Model )


pardon my english