gistfile1.cs

var orders = new [] { 1, 5, 7, 42 };
var customers = new [] { 1, 2, 3, 4, 5, 6, 7, 42 };
var customersWithOrdersCount = 0;

foreach (var c in customers) {

    foreach (var o in orders) {

        if (c == o) {
	    customersWithOrdersCount++;
            break;
	}

    }

}