Kubra, can you provide more context, please? This may require more than a simple translation--you may need to frame it in the proper cultural context for your Japanese guests (If I am jumping to a wild conclusion, I apologize in advance;)) 

Were you trying to communicate to Japanese guests that you may be unable to accommodate their requests due to the high number of guests??? If that's the case,  apologize to them, because お客様は神様 and Japanese always apologize even if they didn't do anything wrong. Saying "beggars can't be choosers" will be a confusing joke that goes over their heads at best (Japanese don't find satire funny, so they'll laugh out of politeness when they realize you are trying to make a joke) or deeply insulting at worst("beggar"「乞食」 is a taboo word that's banned from TV broadcast). They likely won't let on that they're insulted; instead they will freeze you out and try to avoid future interactions with you.

  A more appropriate phrase might be 「申し訳ございませんが、いらっしゃるお客様が多い為、選択が限られております。ご理解いただけますと幸いです。」, "I am deeply sorry, but the selection is limited due to the large number of guests. I greatly appreciate your understanding."　A native speaker can do a better job of tweaking the above sentence to fit your needs.

If I was wrong about why you need "beggars can't be choosers" in Japanese, go with David's advice because it's the weblio.jp definition.   Aki's advice misses the mark and actually means more like "he who does not work, neither should he eat", or "a hungry person will do anything for food", so avoid that one unless you are trying to get your guests to mop the floor in return for food.