I've read the following phrase on a website:

Instead of adding new fields to a content type, adding existing fields is a better option to reduce the system’s complexity and to improve scalability.

And some doubts arise.

In the system we are developing, we have the possibility to reuse a field across 3 or 4 content types but instead of improving scalability as the quoted phrase says, I'm afraid it will diminish it, because the field's table would faster become a bottleneck (at least that's my reasoning in this case, as all values of that field together, would be a couple millions per year and that would make the table too big). Do you agree?

How many rows would be a sensible maximum to aim for when architecting? That way we could decide when to reuse fields and when to create new ones (even though the chance to reuse is there).