Best aproach to store Remember Me data

I was reading the same article☻☻ -
I didn't get the whole algorithm

Am i right - that selector - is just an unique ID (and we must use selector to add a little bit secrecy about amount of users)

So my table must have such structure (bare minimum)
`id` integer(11) not null UNSIGNED AUTO_INCREMENT,
`selector` char(12),
`hashedValidator` char(64),
`userid` integer(11) not null UNSIGNED,
`expires` datetime,

how to generate selector to exclude potential hash collision?

My main question is - do i need to generate a new selector and hashedValidator, each time a user logs in and add a new entry in database.

So i mean if the same user logs in using different browsers - in cookies he will get a different cookies (that have different selector and hashedValidator)

do i need to make only one entry for user (in firefox and chrome user will get the same values in selector and hashedValidator cookies)