BBQ competitions are won by the people that can handle whatever hurdle is thrown at them that day and turn out product as good as when things go perfect. Because they rarely go perfect. Something takes longer than expected, temps don't run as smooth as normal, rain sucks the heat out of your cooker, etc. You have to be able to adjust, and part of that is having a plan B (and C, and D).
This! ^^^^ The difference between the experienced teams and the new teams is how you react when chit happens, and it will happen!
But, to get back to the question on cooker space, it really comes down to timeline and how much you cook. As mentioned, it is possible to cook a comp with one cooker if you manage time right. I'm not sure that I would want to start out like that, however.
To me, a great starting point is a couple of drums and a Weber Kettle (or similar). Use one drum for brisket and one for pork, and time it so that one, or both, are done in time to put the ribs in, and use the kettle for chicken. You could get by with two drums, but a kettle is cheap, especially off of CL or FB Marketplace, and is good insurance.
This past weekend we wanted to support the local Chamber of Commerce and their People's Choice so I cooked an extra butt, and left the butts bigger than normal. I didn't like the way that they fit on one drum so I took my chicken drum, and made it a second pork drum and started the pork earlier. When it came time for chicken the pork was resting comfortably in the cambro :-D
Visit a competition or two, look at the cookers, talk to the cooks to see how they use their cookers, and then decide what works best for you.