Shift registers. What, how and why? I'm making a project and I'm using multiple 1 digit 7 segment displays. I haven't plugged them all on my arduino because currently I'm just testing. The problem is on the I/O ports. Obviously I need to use shift registers and since it's my first time ever using them I need to know more about them. After googling, I found that the specific model I bought outputs 8 pins and you can stack up to 8 in series. So lets do the math. 8 shift register x 8 pins each SR = 64 new pins It seems cool but assuming I'm using 28 displays let's do the math again.
28 displays, each display has 8 pins (8 for segments and 2 as common cathode)
28 x 8 = 224 required pins.
Is is possible to expand the I/O pins by 224 pins? How many shift registers can I stack together? Also which is better, the single digit display or the 4 digit display?
You can chain as many shift registers as you want.

Most commonly, these displays are multiplexed. Only one digit is lit at a time, cycling though all the digits fast enough, so the eye can't tell. This is however not very good practice for 28 digits, as the brightness will become 1/28th (though you could create groups of digits, and multiplex those groups separately).

There are specialized chips that do this multiplexing for you. E.g. the Max7219. This chip can multiplex 8 digits, with 8 segments. So you'd need 4 of these to light up 28 digits.