Rule 8.06 states that "A professional league shall adopt the following rule pertaining to the visit of the manager or coach TO THE PITCHER. . ."(empahsis mine)
Rule 3.03 states "A player, or players, may be substituted during a game at any time the ball is dead." Furthermore, 3.03 gives a scenario in which 2 or more substitutes of the defensive team enter the game at the same time.
Based off these things, I deduct that 1) it is not illegal to conduct multiple substitutions of the defense as long as it is during a dead ball. 2) Visits relate only to the conversations between the manager/coach and the pitcher (including by way of the catcher/infielder - Rule 8.06)
As the PU, I would kindly ask the coach what the heck is going on. But that is just me.
