申请一次可以一直使用. It is like the social security code that once assigned to you, it is always the same number. 怎么能证明父母呆够了183天? I think it is on the honor basis. If there is no room to fill the information on 1040, you don't have to fill it. If IRS come back and audit your 1040 form, you have to prove to IRS that your parents stay in US longer than 183 days.