What's number is the episode .where in the open scene gargamel is chanting from a book in front of a fire and he says "ravage this land as never before total destruction from mountain to shore

Its actually from a special episode, The Smurfic Games. Here is the second half of the special, at 8min57sec he finally says it. (He tries unseccessfully many times throughout the episode to get the chant out... )
Smurfs S5 - The Smurfic Games 1x2 SPECIAL - YouTube