I don’t think a rule like “to count as a lane, it must be at least X meters wide” is feasible cause what is considered drivable will also differ quite a bit from country to country. I wouldn’t tag a residential road that is not broad enough to let two trucks pass each other without evasion with lanes=1 if the normal traffic on that road can do so easily. My rule would be, how the road is actually used. So, in doubt, checking it on-site: Do oncoming cars generally pass each other without having to evade each other or not?
This is not too objective, I know. But a measure that is objective is width, we have that as well. So to be precise I guess it would be better anyway to tag the width instead for slim lanes < 3 roads.