Correct!
Dr. Maiju Sarrat, Mercedes Munoz, and the last unknown sender, who you can assume is the person in the gravchair, as written. That makes five.
1 or 2 more, depending on how you look at it as they are tied together even though they are mentioned separately in the prologue. But, specifically a clue is in chapter 1 that will point to the clue in the Prologue. Once found will bring into question the last undercurrent. Both can be either seen as separate or together. Hence the 1 or 2 more statement earlier.