The use of the word "church," in this context, can easily trip-up modern readers. The word "church" in ancient languages meant "assembly," "company," or "congregation." Thus, in Greek, the word ecclesia (often translated as "church") simply implied a gathering.
In Nephi’s vision, we learn that there are two gatherings and that there are only two. You are either in the Church of the Lamb of God—the assemblage of the Lamb of God—or in the church/company of those aligned with Satan. We can’t always tell who’s on the Lord’s side, who? It will ultimately be the justice of God that will finally separate all people into one of two groups in that last day. So it is not for us to say who will ultimately be in one group or the other. Nephi wrote chapter 14 with an apocalyptic view, which pertains to the end times and shows how things are going to conclude. In the end, there will be only two choices. You either are with the Lord or you are not.
What did Lehi see as the great and abominable church? Lehi called it the great and spacious building. Anybody in the great and spacious building who points fingers at and mocks those who are righteous is in the great and abominable building or church. The word "great" means "very." So, it is very abominable and very spacious. It is a big building filled with a large crowd.
In contrast, what did Nephi say about the gathering of the righteous? "And I saw that they were few." This is true. There may be more than 16 million people who are members of the Church today, and we are happy for it. But 16 million is a drop in the bucket when compared to the entire world population. That is not to say that a little leaven cannot leaven the entire loaf, or a little salt cannot season an entire pot of stew. But the word few still tells us something about the demographics of righteousness.
Book of Mormon Central, "Are There Really Only Two Churches? (1 Nephi 14:10)," KnoWhy 16 (January 21, 2016).