what number is considered a high density carpet?

The density number system is one invented by the carpet mill.

They rate their own carpet, which means the number is meaningless.

Even if it did mean something it would only be a small part of the carpet quality equation. Type of fiber(wool nylon etc), type of construction(tufted or woven), quality of backing material in the carpet itself and other factors weigh heavily in the determination of quality.

Do not trust any type of number system whether it is weight or density. It is smoke and mirrors.

Find a dealer who you can trust and ask for their advice. If he mentions the numbers without explaining any other feature he probably is not knowledgeable and/or has an angle.

Ask friends where they get carpet, check with the better business bureau, and/or see who does their installs(employees or subcontracted out.