History shows the following. I went back further than planned, as the data interested me. Data is courtesy of the National Federation of High Schools Participation Survey, as well as a couple state wrestling sites.
1970-1980: Rapid growth, from about 7000 High School programs nation-wide, and 225,000 or so total wrestlers, in 1970, to a bit more than 8700 programs and nearly 275,000 wrestlers in 1980. Data shows, on average, 35 wrestlers per team. Several years show over 9000 teams and the number of wrestlers exceeding 300,000.
Each state sets their weight classes, and in this time frame, as examples, Ohio went from 12 weight classes to 13. If my info is correct, Pennsylvania was at 12 the whole decade.
1981-1990: Number of teams stable, at around 8500. Number of wrestlers (average) per team dropped from the 35 range in the previous decade, to 28 per team in 1990.
Ohio went from 13 weight classes to 14 during this decade (which is where they stand today), and Pennsylvania went from 12 to 13 in 1989.
1991-2000: Total number of teams nation-wide increases during the decade, to about 9000, though the average number of wrestlers per team decreases to about 26.
2001-2010: Team count continues to grow, as there are around 10,000 nation-wide. The number of wrestlers per team holds steady compared to the previous decade, at around 26.
Pennsylvania moves to 14 weight classes early this decade, in 2003.
2011-Present: Number of teams grows slightly, to today's 10,600+. There is a decline in the number of wrestlers, from 26 (previous decade+) to 23.
Pennsylvania: During this same time-frame, PA wrestling (in the 1970's) had 550 to 650 or so high schools with wrestling programs, and a range of 21,000 to 28,000 wrestlers, or about 40 wrestlers per team. The 1980's and 1990's saw the number of teams stabilize in the 470 range, though the number of wrestlers per team plummet from 40 (in the 1970's), to about 20, where it is today.
Discussion: This is complicated. There's far more data to consider than just what is above. The numbers are averages, so for every team with numbers of wrestlers above 20 (PA) or 23 (nation-wide), there will be teams below. When thinking about 14 weight classes and average number of wrestlers in the 20-23 range, it's not hard to understand the number of forfeits, though it's only one side of the argument.
Just since 2012 (to 2017), the total number of High School Wrestlers dropped from 272,149 to 244,804, or 10%. The PA numbers held steady in that time, while the national numbers did not.
As an aside, there were 146 D1 Wrestling programs in 1982, and 76 today.