Do oviparous snakes and lizards always lay multiple eggs or are there any species which produce a single egg at a time?

There are some geckos like Thecadactylus rapicauda, Gehyra variegata and Ptenopus garrulus which lay one large egg. All Anolis lizards also produce single eggs, although they can produce them at 7 day intervals. This is quite unusual for squamates though - most have clutches ranging from 6-50 eggs.

See page 111 of Pianka, ER & Vitt, LJ. 2006. Lizards: Windows to the Evolution of Diversity. University of California Press.