I am creating a form which provides me order from my customers

for this i am use two Forms first form for registration and another for Create Order

on the registration time i am provide 50 Orders to my every customer. this mean my customer give me only 50 orders. 

Now in my Create order form there are following fields

1. customer name

2. date

3. total Orders -50 default set on load

4. Rest Orders-

5. Order

6. email ID

so now i want that in rest orders filed the customer sees his rest orders automatically. means If a customer give me a his first order then when he come back for next order he sees the Rest Orders field-49 orders. when he again comes then it shows 48 and so on....

so please tell me how can i do that.