It depends on many many factors. IE;
What is your application? Off shore racing, lake racing, bass tournament
fishing, water skiing, trolling for fish, running a charter boat, gentle
curse on a small quiet lake?

How long is the boat? 8ft, 80ft, 800 ft?

How much is your budget?

Factory built outboard boats built in the past 40 years or so have a name
plate showing the maximum rated horsepower.

You might get a better reply at your local boat dealers.

If you can be more specific, we might be able to offer some suggestions.

Don Dando