So, with the following factors in mind:

1. we consider a crossover the interaction of characters not owned by the same person/organization
2. we are only counting one-shots and limited series (my db does not track the month every issue came out and inter-company crossovers in ongoing series are a minority)
3. these are only the ones I have purchased (but I will get anything that looks like a crossover),

here is a quick graph of titles (so a one-shot and a whole limited series each counts as one) which feature a crossover organized per year:

And some people still have the balls to say that comics in the 90’s were bad!!!