How many aircraft types can a 135 pilot be assigned to? Is there a limit to how many aircraft types a §135 operator may assign a pilot to fly? In other words, could a pilot type rated to fly—for example—a Learjet 75, Citation X, and Challenger 300 be assigned by the §135 certificate holder to fly all three of these types in §135 operations at any given time? Would that pilot be limited—for example—to only two types at a time for the purposes of §135 flight operations? I am not considering flight assignments that are to be conducted under §91. Is there a difference if the pilot is assigned as PIC or SIC?

I ask because my understanding has been that a §135 operator may assign a pilot to a maximum of two aircraft types at a time. However, I cannot find any documentation or discussion to support this.

Edit: To be clear, I am not asking about how many type ratings an FAA certified pilot is allowed to acquire in total (the answer to that question is: no limit). I am asking about what §135 operators are authorized to do, as far as assigning flight crew to various aircraft types.
There is no FAA regulation limit for the number of types a Part 135 pilot can be assigned to, providing all training programs are followed.  Insurance and safety auditing companies usually have guidelines requiring no more than 2 types to be assigned at a time so the 2 aircraft limit has become something of an industry standard.

There may be non-US regulations that address this issue.