Can there be multiple Blue Shells at once?
No, there cannot be more than one Spiny (Blue) Shell held by players at any given time. This mechanic was apparently introduced in Mario Kart: Double Dash!! and has not been changed since. From the Spiny Shell wiki page:
Additionally, once a player holds a Spiny Shell, other racers cannot receive another shell until the player has used it and hit the target. This rule is kept for later games as well.