It went through multiple formats over the years.

From about 1988 until expansion in 1992, there were seven games. You played five perennial opponents, and the other four rotated through the other two spots, so you played everybody at least twice every four years. This was great.

Before that, from sometime in the 1970s until 1987, there were six games. You played five perennial opponents and the other four rotated through the other spot, so you played everybody at least twice every eight years. This sucked, but not as bad as the current system where six teams rotate through one spot.

Before that, it was pretty random. SEC teams simply scheduled whoever they wanted. Teams went decades without playing each other (LSU and Georgia didn't meet a single time from 1953 through 1977, for example). Teams also didn't play the same number of games. Some teams played as many as seven SEC games, others might play as few as five. One year, I believe in the late 1960s, LSU actually only played four SEC teams, and a game against TCU actually counted as an SEC game for us just to get us to five. In some years, the difference in the number of games two teams played made the difference in who won the SEC title (e.g. 6-1 record vs. 5-1).

It was a mess for most of the history of the conference. Finally, in 1988, they got it right, but then they screwed it up four years later with the first expansion. Again in 2002, when they went to only one permanent inter-divisional opponent and two rotating ones (so you played everyone at least twice every five years), they got it back to about as good as it could be with divisions, then screwed that up again ten years later with another expansion.