Welcome to AC Adrian,
To answer your question no that won't buy all the live rock you'll need for a 55 gal. You'll need to get at least 1.5 to 2 lbs per gallon,"unless it's very,very porus and lite. then you might get by with 1lb." Don't despair because you can always make your own live rock. Just buy dry coral rubble from a lfs or off line and at least some uncurred live rock. Run your tank through a cycle ," use either ammoina or a cocktail shrimp etc to raise your ammonia level to 3 ppm and allow your bacteria time to spread.
It might take it a while for it to purple up and look like live rock but, after you get your bacteria colony established you can start adding stuff. If you have any more questions feel free to ask.
hth
Max
