However, modules sometimes assume that you have all earlier modules and therefore have all previous overlays. And TPPs use overlays. So, I'm looking forward to the overlay pack.
Quite, quite true.
I was more thinking about the indexing & source end of things. If you can't get your hands on some core modules, then I would imagine that their contained overlays might be the least of your problems. While I occasionally wonder something like "I have 10 T-34 m43, where did the extra 4 come from, should I have 12?", in that case it would be CoB and no, 10 is the correct number, but overlays are just a question of "Do I have them or not?".
When designing either for eventual publication or DYO, then the number of a unit type available might be a constraint, however I don't remember any scenario with using more than a single copy of any individual overlay. Some of us bought multiple copies of modules, especially in the late-'80s and '90s when there was little else ASL to spend on, so what we have will be different to what someone with 1 of each module would have. Our extra overlays, given their sparse use, most likely remain uncut.
I could tell you how many Soviet (and likely German, French, Italian, Japanese, Axis & Allied Minor) vehicles that the 1 of each module player should have, but I have mentally lost track of US and British, especially Sherman sub-types, because of the bits and pieces in places like OVHS, Elst, etc, etc.
Counters are "How many do I have?", overlays, maps, rules are a case of "Do I have it Yes or No".