First, start by placing 7 segments in anything that is visible enough when activated/actuated/witched on [that's not so hard]. I like gemspark blocks.
Then you just need to wire on the segments that need to be switched off and it's done...





But that's too easy! That's cheating! And it's impossible to display more than 4 numbers [there are 4 wire colors]!

If you wanna display absolutely everything you want ! First make the 7 segments and use one wire color per segment. Yeah, I know, only 4 colors for 7 segments, but just try to avoid using the same color for 2 adjacent segments :D [i've said too many times segments today].

To recognize each part I'm gonna use letters like real 7 segment displays



You can then connect each segment to a lever or a switch or actually everything and anything [but be careful not to cross any wires]




And if I switch on segments B, C, F and G for example... It displays 4!

Well... Personally, when I want to display the number 4, I prefer using a switch [or anything] for the number 4, and not 4 different levers for 4 separated segments! I said 4 exactly 4 times in the last sentence, I hope you're not supersticious!

One of the ways to display a number is to use minecarts. That's really easy!

Instead of levers, use pressure plate tracks for each needed segment for your number, and then use your cart!

Here, I will switch on B, C, F and G segments, and guess what is going to happen?

Number 4 :D



You can align vertically many of these structures to display absolutely anything you want !


Even letters...

If you don't know what is a hoik, it's er... too weird to explain. With some placements of "teeth" the game just decides to make you move super fast for no reason. But that's soooooo funny!


Just remember that "this hoik" will make you go super fast from left to right: just go through the stairs pressing down key, then hoik !

You can then place pressure plates as the minecart part !

Logic gates can make many things easier, faster, more efficient... they are great!

As you can see here, on the left side wires are connected to E, F and G segments, and on the right side wires are connected to A, B, C and D segments. Probably, you won't have the same thing as I had but it's not that bad.


Now, we're gonna place two logic gates for the two groups of segments; if we used only one logic gate, wires would cross and do anything!



With that we can switch on or off the logic gate. The logic lamps which are always on are here just for spacing and prevent wires for overlapping.

Then connect only the segments I need to the logic gates...


For example, for 3 we connect on the left side the G segment (red wire) and on the right side A, B, C and D (all colors)



You can add as many number or letters as you want, and this is really more compact than minecarts or hoiks!

Good luck!
[well... it was actually meant to be ";)" ]