How many minerals are know to occur in the Earth?

This is debated, and can be answered a few ways.

I think it's safe to say that no more than 300 of the over 4,000 known minerals can really be considered common. The average collector will get to know them, as they are all available in decently sized crystals. See Mineralogy by Sinkankas.

To a geologist, though, well, he wouldn't be interested in more than 100, if that.

From the serious collectors point of view, if you have the money and the motivation, it's no big deal to obtain over 1,000 different mineral species. 2,000 is a challenge, and over 3,000 is what separates the men from the boys, where you're paying $300 + for polished sections with tiny specks that can only be confirmed by SEM.

The average collector, though, if the shows and magazines are anything to go by, is more than satisfied by the top ten or 20 minerals that come in big, colorful crystals, like quartz, calcite, tourmaline, beryl, etc.

Probably a longer answer that you're looking for. The answer in the Geology 101 textbook, would, I suspect, be 100. Hopefully somebody will correct me if I'm wrong.