How many eggs does a boa constrictor lay?

None.

Boa Constrictors are ovoviviparous which means they give birth to live young.

Boa constrictor embryos develop within an egg inside the females' body and hatch shortly before birth.

It seems the number of offspring produced can vary greatly with claims of between 10 and 60 hatchlings per clutch. I would say that around 20 - 30 viable hatchlings per clutch is typical.