I would suggest the following:
1 - If your Customers type is not what I changed to Customer in my example, then, you should make your customers array an array of Customer ([Customer]). By the way, you don't need the initializer call since you are initializing the array in your initializer. You can get away with var customers : [Customer] and you don't execute any code since you are going to actually initialize later.
2 - Not sure what getAllCustomers does, but, I'm assuming you are fetching rows from your database and initializing Customer instances. If you are using the Customer type, when you create an instance with two strings, you'll get an instance of Customer with a custNumber String, and a custName CustName or Customer.Name depending on whose example you use. Key point is the the custName property has the < and == operators defined on it that you can now use in your sort call. By the way, you should probably figure out what to do with if you get two names that are equal.