How does Ripley get face-hugged?

Rating: 2/10 (13 October 2005)

During the introduction of the film we see that there is an alien egg behind a wall with a face-hugger coming out. Technically one face hugger means one alien, right? So how come that in the film we see Ripely discovers she has an alien inside her and the dog getting transformed into an alien, when there is only one egg on the pod.
Because that in alien(1) Kane gets infected by the face-hugger and it dies after it implants the egg in his body. So surely this means there must be 2 eggs or 2 face-huggers aboard on the spacecraft, but we only see one. Also in Alien(2) we don't see the queen alien lay any eggs on the aircraft which saves Newt and Ripley.