Hi Eddie,

Auto-Res has 17 services a day from wich 10 are Express and 7 are Normal.
So I don't think you will not have any problem unless you are traveling on a friday, the last day of July or the first of August.
People leave Madrid en masse those days for weekend or for the August holidays.
Anyway you can make a reservation online, even you can buy the ticket with a credit card.
Here is the link:

www.auto-res.net

Enjoy your holiday