There is absolutely no way for you to know the food allergies and dietary restrictions of all of your wedding guests. And you should not take a poll to find out.
Your caterer is a professional and he will know how many vegetarian meals should be made based on the number of guests you are having and his experience and years as a caterer.
As for guests with gluten, peanut and other allergies, they'll know what they can and can't eat and won't be afraid to ask the catering staff if the food contains whatever it is they are allergic to.
If you happen to know that there are some guests who are kosher or vegetarians you can give your caterer a heads up to prepare special meals for these guests, but you do not have to reach out to each guest in advance of the wedding and find out what he or she will or won't eat.