Condom broke day 21 out of 28-29 day cycle . i took plan b 8 hours after incident what are chances of pregnancy ?

Almost: Nil as that was not your ovulation or fertile time. But anytime you have an unprotected encounter check a pregnancy test if late.

Very less: Normally fertility period ends around the time you mentioned. As a precaution you took plan B. I agreed. I think it will take care the issue. However if you missed a period its always better to do a pregnancy test with early morning first void urine.