The funny thing is that every number you have ever had is still linked to your account for mileage earning purposes.

I'm sure sure of the exact amount, but between normal M&M cards, status changes, credit cards, credit card replacements, and the like, I must have something like 20 valid Miles&More numbers.

Why this should be the case and why LH can't just assign you one fixed number like practically every other FFP under the sun is beyond me. Especially since they already have something like it in the form of the customer number.