Which creature can lay largest number of eggs?

well, turtles and lizards mostly. they can lay up to about 150 eggs.

Some toads lay tens of thousands of eggs. The cane toad, according to Wikipedia, lays from 8000 to 25000 eggs at once in strings that can reach 20 meters in length.

Ducks, geese, and other waterfowl that are often visited by nest- robbing creatures often lay as many as 20 eggs in a clutch in order to make sure that some of their eggs will survive.

a woodpecker replaced 71 eggs in 73 days when researchers tested how many eggs she would lay.