I hadn’t noticed, but apparently Bobby Draper was played by eight different actors over the course of the show. (Also surprising: two actresses played Sally, though the other girl was only in the pilot). Some were apparently one-offs, and the last two carried the bulk of the episodes, which is probably why we as viewers didn’t really notice.
Because I was curious, I looked it up. According to IMDB, the character appeared in 74 of the 92 episodes (Sally is in 89). * The last Bobby, Mason Vale Cotton (b. 2002), had the role for 33 of those appearances. * His immediate predecessor, Jared Gilmore (b. 2000), was in 19 episodes.
That leaves 22 episodes where someone other than these two played Bobby.