There are normally two readers scheduled for Sunday Masses and most holy days of obligation. Normally, there is only one reader for weekdays. However, a lot of the time one does not show, or sometimes both readers do not show. Father likes when there are two readers on Sundays, but sometimes only one will suffice. Always expect to be pulled from the pew if someone does not show!
A couple months after I had started reading, both readers did not show, and I had to read for the both of them. I was so nervous, and honestly, my presentation of the readings was horrible! I was so embarrassed. After that experience, I always familiarize myself with the readings of the day just in case I get pulled at the last minute.