I recently returned from a trip to the Czech Republic and Austria, and spent a lot of the trip in more historic areas of the city. While there, I noticed that the churches (all the same denomination, Roman Catholic) were very close to one another - for instance, Prague had about half a dozen very large churches within a 5 minute walk of the old town square, without including things like small chapels in other buildings. Of course, some are older than others, but at some point all of these churches would be in use simultaneously. This made me wonder - did people living in these areas usually go to the same church, or alternate between them? Were there particular social conventions around who went where (i.e. all of the aristocrats go to one church, another is used by merchants, etc)? Were the churches in a sort of competition with one another?
The short answer is that people could choose what church they wanted to attend for mandatory Sunday masses though this was not looked on favorably by parish clergy. It was only in 1517 that Pope Leo X ruled that Catholics could hear mass in whatever church they liked.
The long answer: The clergy resisted this freedom of choice because of the regulations of the parish church. With a few exceptions, each church in a town was the central building of the ecclesiastical unit known as a parish, an innovation of the Middle Ages in place across Europe by the twelfth century. Canon law, the official law of the church, had established over time what came to be known as the jus parochiale (“parish rights”), which Christians born within the boundaries of the parish were obliged to observe. This meant that they had to be baptized, receive annual confession and communion, and be married in their parish church. They also had to be buried within in (if wealthy or nobility) or in its surrounding churchyard. They also had to pay tithes—one-tenth of all the things they produced—to the rector of the parish. (With permission, they could receive confession and communion from another priest outside the parish, be married, or buried outside their parish.) In addition to tithes, there were various other “offerings” parishioners were expected to give to their parish priest, usually at Christmas, Easter, and a few other holidays. Corporately the parishioners were bound to see to the upkeep of the nave of the church, that portion where they stood during mass. In other words, the parish was a spiritual unit but it was also an economic entity so parish priests carefully guarded their rights. Thus their resentment of their parishioners "parish hopping" to hear mass. In larger towns, there could be dozens of parishes. Medieval Norwich had over 40 parishes for a pre-Plague population of about 10,000.
Some of the churches in a town belonged to monastic groups, some of which sometimes doubled as a parish church.
As towns expanded their boundaries beyond the Middle Ages, new parishes were created.
I’m pulling this off the top of my head; but Katherine L. French’s The People of the Parish: Community Life in a Late Medieval English Diocese summarizes the general picture. Bernard Hamilton’s Religion in the Medieval West offers a much briefer view.