I have connected up to 3 chips to the 8051 external bus, and it works well. My project runs at 12 MHz crystal frequency. I think that higher the frequency, lower the number of chips that can be connected, because the pin capacitances.

If your project is more than a toy, you should test it over the whole temperature range and, if possible, with different MCU samples.

Regards,

Alfredo del Rio.