oracle table update question

 our customer have some question about update to ask!

1. a table have 5million rows , they want to update 500 thousand rows , so it will have row lock,

they want to know if there will have 500 thousand  row lock ( one row lock  per one row), or

just one row lock  ( one lock per 500 thousand rows  ), they want to kown oracle lock mechnisam.

suggeust just  update two row , how oracle hand it , first lock one row, then the same lock  lock the second

row, commit, the lock will release , is that  explain right  ?

2.how is update ( update  numbers  of row )  or the lock  effect to db , can cause oracle problem,can oracle hang!