What are the chances of a girl getting pregnant if she is on her sixth day of her period and a guy cums in her?

Unlikely. Unlikely. However, it's possible that what is thought to be day 6 of one's cycle may be incorrect and thus, chance for conceiving may be higher.
Varies. Pregnancy has been documented from unprotected exposure on any day of your cycle including during menses, although less likely at that time. This would be more likely in someone with erratic cycle times and less so when they are quite predictable. Low means low, but that also means it is possible.
LOW chance. The chance of getting pregnant on menses (period) is low, but is still possible, so please take precautions.