How many masterpage we can add in a project

Yes.you can have multiple master pages in your project

http://i2.asp.net/asp.net/files/DataTutorials/datatutorial03vb.pdf?cdn_id=2012-03-01-002[^]

you can have as mnay number of master pages as you need in your project, but only one for single content page ,
but you can change it at runtime depending on some condition, this is generally done in page_init() by writing

this.MasterPageFile="master page path".