In the time of St. Ambrose (fourth century), we find the custom of having more than one altar in a church; and St. Gregory (sixth century) evidently approves of the same by sending to Palladius, Bishop of Saintes, France, relics for four altars which, of the thirteen erected in his church, had remained unconsecrated for want of relics. After the introduction of private Masses the necessity of several or even many altars in each church arose. They were erected near the principal altar or in side chapels. The altar in the sanctuary or high chapel always remained the principal one of the church, and the pontifical services in cathedrals as well as the solemn functions in other churches invariably took place at the chief altar on Sundays, holidays, and other solemn occasions of the year.
This is a photo from a church in New York City.