There is info about this in a thread somewhere but IIRC the thread was not originally about this and the topic drifted.

There are about 40K diamonds representing roughly the top 0.05% of MQM earnings in the qualifying year among all SM accounts, of which there are roughly 80 million. There are a couple million medallions, representing about the top 2.5% of MQM earnings in the qualifying year among all SM accounts. The vast majority, roughly 80% of medallions (or about 2% of all accounts) are silvers. About 12% of medallions (or about 0.3% of all accounts) are golds. About 6% of medallions (or about 0.15% of all accounts) are platinums, and as noted about 2% of medallions (or about 0.05% of all accounts) are diamonds.

These are rough numbers, and DL will not reveal anything exact as it is valuable proprietary information.

Of course some people move up or down levels from year to year but the percentages stay roughly the same.

As to OP's question about what level should qualify you for perks from either DL's standpoint or the customer's, that is an age-old debate on FT and you will hear a wide range of opinions. But I can tell you that no loyalty program is about rewarding past purchases, it's about driving future purchases, so DL is going to adjust the type and extent of perks to do so as successfully as they can.