No, every general and subject GRE test has its own 7-digit registration number. Your unofficial pdf score report only shows the number for your most recent test. For example, if you took a physics GRE this April and then took a general GRE this September, the 7-digit number showing up on your score report as "registration number" is that of the September test.
If you want to know the registration number of a test you took and it hasn't been six months since, you can check it through the "GRE Diagnostic Service" section in My GRE. If more than six months have passed since the test, you cannot use the Diagnostic Service and hence can't check the number online; you have to call ETS and ask.
I don't know why ETS doesn't provide us with the numbers on the website, but I believe they've got their reasons. It MAY be okay to put a wrong number on the application, since the graduate school can use your your name & birth date to match scores, but it is to your best interest to call ETS and get the correct numbers.