If the venue is in WA, you can tell the operator to read Governor Inslee's proclamation dated 16 March in which the governor ordered all wedding venues to close for at least 30 days. See https://www.governor.wa.gov/sites/default/files/proclamations/20-13%20Coronavirus%20Restaurants-Bars%20%28tmp%29.pdf .

Like barber shops, nail salons, and a host of other small businesses, there is no exemption to the number of persons at the wedding venues. All venues must close regardless of the number of persons.

It is not you who are canceling the event being held at the venue. The venue operator is the one canceling the event.

On another hand, you should not expect quick help from the court system in getting your money back. Many WA courts have closed or have reduced the number of functions the courts provide to the public.

The first step in interpreting a contract is to read it. Perhaps there is a "force majeure" clause with provisions for the non-fulfillment of a contract due to unforeseeable circumstances. If you have to sue to get your money back, you would have a good argument. Small Claims Court works if the amount in controversy is under $10,000.
https://www.clark.wa.gov/district-court/small-claims

Hi there-

Sorry to hear you're going through this. As someone who was recently married myself, I understand the stress leading up to the big day, much less having to deal with this issue now.

You pose an interesting question. Does you agreement specify your number of guests? If they cannot accommodate that number of guests, they may be unable to perform under the contract and it would provide you with the leverage to demand they perform or cancel on you.

This is one where reviewing the contract would be particularly useful. Send it my way if you would like. erik@halversonlaw.com

A formal letter demanding full refund may be effective and cost conscious for your issue.