For several years, there were so many sets to choose from that you got what you could at the time and then could maybe go back later and pick up others if there were still boxes and singles available. Lately, with the Topps exclusive contract, there is really only one major flagship set, so it's been relatively easy to finish it with a reasonable amount of inserts to go along.
I researched the numbers, starting in 1996, which seems to be when Topps started putting inserts that are independent (numbered separately) sets, into regular packs of flagship. The numbers include inserts that came with Series 1, 2, and Update. I didn't include jersey, autograph, manu-relics, or any low serial-numbered ones, just regular cards. Also omitted oddballs that were packaged separately, or player reprints (like Aaron, Mantle, Mays etc. around the turn of the century) because their number was dependent on the existing cards of that player. I also did not include factory set exclusives or early retail-only items.
From 1996 to 2007, with minor spikes in 2001 and 2003-04, Topps averaged around 125 to 175 total insert cards available. The low point was 2002, with only 112 inserts to partner with 995 base cards. Since 2008, Topps has expanded the number of inserts in its flagship set to almost TRIPLE what it used to average, while maintaining the same number of base cards (990). The total number of inserts doubled into the 400's and then shot up into the 600's by 2010. In 2016, it peaked at 674!
The distribution of inserts has changed as well. Up to 2010, there were between six and nine insert sets produced. (11 in 2007). Then between 10 and 13 for the next few years. 2015 and 2016 doubled that to 23! Base cards only increased 110 for the three series in those years. So instead of inserts being a fifth of the total number of cards, they have become roughly HALF!
Since they doubled the number of insert sets, the average number of cards in each insert set has come down. It peaked in 2009 with counts of 150, 100, 75, 75, 30 and 25. An average of 76 cards per insert set. The average for the first several years is 15 to 20.
I think an insert set is fine with between ten and 25 cards. Any more than that should be spread among all three series, and limited to 75 or so total. Maybe 100 for an original idea that applies to a lot of players. But 100 cards done three times is its own set! It's too much to put in with the base flagship product. Topps keeps putting out repetitive products to fill out their catalog - make some of these inserts into separate products! Then you may actually come out with something with a little innovation! You're killing us completists with 1100 base cards and 675 inserts on top of that! At best, they run 25¢ each in the secondary market.
But a lot of them are recycled versions of the same thing. Mostly annual reprints of famous cards.
2017 Series Two is about to release. What will we be buried in this time?