There is no definitive count because “Grow” is not a single franchise—it's a design label used by various developers (most notably On / Eyezmaze for the GROW series: GROW ver.1, 2, 3, Cube, Island, Valley, Cannon, RPG, Maze, Tower).
Eyezmaze's GROW games all feature secret/optimal endings triggered by finding the correct item-click order (usually 1 specific permutation out of hundreds). Each game has 1 “Perfect” ending + several “Bad” endings.
Other games with “Grow” in the title (e.g., Growtopia, Grow Empire: Rome, Grow Castle, mobile idle games) have different mechanics—some have hidden achievements, not narrative secret endings.
To find secrets for a specific GROW game, consult Eyezmaze's site, Jayisgames archives, or YouTube “perfect order” guides.