Measure I2C bus capacitance
The answer is "it depends." It depends on how strongly each device can drive the I2C lines; on how small of pull-up resistors all can tolerate; on what speed the I2C is clocking at; and how much parasitic capacitance is induced by the bus wiring length and topology. Smaller pull-ups, slower clock, and shorter (external-PCB, good spacing) bus distances all reduce capacitance and thus allow more devices. Even prototyping on a solderless breadboard is not ideal, as the pin connections will likely introduce significantly more capacitance than say, a 2-sided PCB will. All of these aspects could be calculated and/or simulated, but Renesas has a good point in that "testing it" may be the fastest and most accurate way to arrive at a real-world answer.