how long is a hen broody for?

My leghorn has been broody for a week, she is determined to stay on the nest despite me trying to discourage her.
Having read the advice on here I am thinking of putting some fertilised eggs under her. I have a seperate hen coup & run so can keep her away from the others.
I have some questions please as I am fairly new to chicken keeping...
How many eggs should I buy? Am I too late after her being broody for a week already?
Where is the best place to buy fertilised eggs?
I would like 2 more chooks & can easily give boys away.

My hen had been broody for 3 weeks when I put six eggs under her.  We are now on day 19.  Last year she was broody for 8 weeks which is why I risked it so late. 

Thanks for your comments, I will keep you posted once I decide what to do