The image holds in the human eye if the rate of display multiplexing is fast enough. Below about 24 Hz you can see flicker. In real actuality each digit of a 5 digit multiplex scheme in on a total of 20% of the time. For LED multiplexing the amount of current probably is higher than what port pins can handle so the 8051 pins need to be buffered. Most often NPN transistors work for the common segment drivers (or use a UL2003 chip that has 7 NPN drivers in one DIP). The digit drivers can be PNP transistors to source current and switch for each digit. It is also possible to reduce the number of pins by using 3 port pins in a binary code and have that drive a 74HC138 3->8 decoder which in turn drives the PNP digit drivers. (If you look around on the web you can find a type of chip called a "high side driver" which can do all the digit driver function in one package. Drivers of this type can be had for 4, 7, and 10 outputs.
Michael