That's just it. Merlock has unlimited wishes because he has a magic talisman. It's what gives him all his powers. And when he puts it on the lamp, he gets as many wishes as he wants.
14 seconds sound clip from the DuckTales: The Movie (1990) movie soundboard.
You can hear this line at 00:29:53.485 in the Blu-ray version of the movie.
Quote context
[...]
- No, bad wisher. You don't know.
- He made me do the worst things.
- And poor Pompeii. Mount Vesuvius would have never blown its top if Merlock hadn't blown his.
- But what are you worried about? He used up his wishes.
- Now do you see why I'm a little jumpy?
- Well, maybe we should wish for the talisman.
- That's the one wish I can't do.
- You'd have to steal it from him yourself, and good luck.
- Don't worry about that mean old master now.
[...]