How do I assign a List (SOQL) value to a variable

I have 2 goals here:

• get a List Value and assign to a variable.
• If record not found, do nothing, go to the next record. (This works)

if query without a list, but I get an error when a record is not found and it won't go to the next record. For example,

Account acc = [SELECT id, OwnerId, Name, AccountNumber FROM Account where AccountNumber =:ReferenceId];

Load list through query: (does not throw exception if record not found)

List<Account> acc = [SELECT id, OwnerId, Name, AccountNumber FROM Account where AccountNumber =:ReferenceId];

Access id value code (Does not work)

newcase.AccountId = acc.id;

Error:

Variable does not exist: id

Regards,