I think you’re asking about individual First Communions throughout the year as opposed to a single large group each year?
Our parish used to do that when it was smaller. Parents would simply notify the priest when they felt their child was ready, the priest would quiz the child, and the child would have his or her First Communion during the regular Sunday Mass. Our son actually had his First Communion in February.
Now that our parish has grown quite a bit, our pastor has decreed that all children receive First Communion on the same Sunday, though the parents can choose which of the three weekend Masses their children can receive. We don’t have a special Mass for First Communions, but I know that a lot of big suburban parishes do that, because otherwise there’s no way they could accommodate everyone who wanted to attend. My nephew’s First Communion group was over 80 kids.