Did you have all the information which needed to be on the poster before you started your beautiful and original design? Or did you create a design, present it to the client, and then get 10 more pounds of potatoes to shove into the 5-pound bag?
If you started with all 30 logos and only designed for six, that's your problem. If you started with six and the client gave you two dozen more, then you have to accommodate the client, not your design, and re-do your design to make them fit.
Unfortunately, sometimes there are requirements which supercede aesthetics. If all the entities need to be represented because $$$, then it's your job as the designer to get them on there. It may in fact look cluttered and dreadful. There's only so much you can do to ameliorate it.
It's not up to you to decide what information is important on a poster which advertises an event. It's up to you to figure out how to make it all work. That sometimes, nay often, means chucking a lovely piece of art into the Keep For My Portfolio folder and creating a Thing That Works instead.
Particularly since these are monthly posters and not corporate IDs or permanent websites, just grit your teeth and do your best. There are times when you can push back and explain to the clients that the clutter is obscuring the message. This is not one of those projects.