Create Request - Response class as per requirement.

public class MyOrderCountRequest : ServiceRequest
{
    public decimal MinAmount { get; set; }
}

public class MyOrderCountResponse : ServiceResponse
{
    public int Count { get; set; }
}

public class OrderController : ServiceEndpoint
{
    public MyOrderCountResponse MyOrderCount(IDbConnection connection, 
        MyOrderCountRequest request)
    {
        var fld = OrderRow.Fields;
        return new MyOrderCountResponse 
        {   
            Count = connection.Count<OrderRow>(fld.TotalAmount >= request.MinAmount);
        };
    }
}