Especially on Saturdays and holidays or when the weather is - unusually - uncomfortable, it gets really crowded in the tents at the Oktoberfest, up to temporary closures due to overcrowding. Of course, it is practical to know in which tent there is still space. You can get this service here with the official occupancy display for the tents on the Oktoberfest.
Everything else you need to know for your visit to the beer tent you can read here. You can also get the tent occupancy in the official Oktoberfest app.