It'll probably be 4 episodes long total like the previous two Don battle tournaments. You're overestimating the length of the battles. Each match is only one on one and many of these can span no more than 3-4 minutes. It was a lot like this in the previous ones.
I imagine Mamoswine Vs. Dragonite will go on for longer due to being an important match, and others like Ash Vs. Trip in the finals. I guarantee something like Iris Vs. Georgia will be over within 3 minutes.