You should check the calculator at Mr.Malty
Mr Malty Pitching Rate Calculator
That will give you an idea of what you should pitch. Probably pitching 2 packs of yeast is still below what it suggests. Don't get me wrong you can make great beer with just 1 pack of yeast but many people will say under pitching is one of the most common problems(weaknesses) a homebrewer makes. Use the search function, there are many threads dedicated to people under pitching their wort. It would take some serious effort to overpitch using liquid yeast and not use a starter. Basically use as much yeast as you want, to save yourself some money make a starter, which the equipment to do so will only cost you a couple packets of yeast.