How to use different 7-segment for different numbers in FPGA board?
You need to sequence through the digits, one at a time, and light the appropriate segments for that particular digit. So you need one 7-segment decoder only and a multiplexer to select the appropriate 4 bits for that digit. Plus a clock and decoder for the digit select and a 3-bit 0..7 counter for 8 digits. 

1kHz is more than fast enough for visual fusion to take place so the display does not appear to flicker, so you might use a 5 or 10kHz clock. 

It's good practice to insert a bit of dead time to allow the current to tail off in the drivers so you don't get ghosting (part of one digit shows up in the next). A microsecond or so should suffice.