Do we ha ve to stop taking vit B12 supplement before taking a vit B12 test? If so how many days before?

No. If you have been taking B12 and previous tests were normal, you probably have a year's worth of B12 in storage. If the purpose of the test is to see if you have normal blood levels of b12, you do not need to stop the oral intake of b12.
No. If you want to see if vitamin effective stay on it. If you wonder if you need it then stop filer a month before testing.