thingamalang = tostring(getreg("thingamalangInput1")) -- so, "thingamalangInput1" is the name of
-- the writer. the number input will be in a string. the writer needs to be named thingamalangInput1
-- now, thingamlang is whatever number the "thingamalangInput1" writer is in string.
-- if you wanted to display this text on a display, use:

-- display = getDisplays()[1]
-- display.clear()
-- display.drawText(1, 1, thingamalang)
-- display.forceFlush()
-- display.flush()
-- and just put this below the tostring(getreg( line

New updated version for 4 digits:
display = getDisplays()[1]

N1 = tostring(getreg("N1"))
N2 = tostring(getreg("N2"))
N3 = tostring(getreg("N3"))
N4 = tostring(getreg("N4"))

ENN = N1 + N2*10 + N3*100 + N4*1000

display.clear()
display.drawText(1,1, ENN)
display.flush()

function callback_loop()
if _endtick then
display.clear()
display.forceFlush()
end
end

so, you need 4 writers named "N1", "N2", "N3", and "N4"
add a counter block, and add the number blocks with the 4 digits coloured for numbers in the thousands. connect the first digit to the "N1" writer, second to "N2", and so on.
add a number into the counter, and turn the computer off and then back on. it will show that number in the display.