Netlogo: How can I obtain the accumulate value in Netlogo?

To fix the issue you can do random 20 might return 0 or 1, and then the first thing you do to move-distance inside the loop is subtract 1 from it, so the move-distance = 1 check will fail because it's already below 1.
Try replacing move-distance = 1 with move-distance <= 1, and/or replace random 20 with 2 + random 18.

Any of those help You need to store the lagged value (e.g., in a turtle attribute). For example,

turtles-own [x xlag]

to setup
  ca
  crt 1 [set x random-float 1.0]
end

to go
  ask turtles [set xlag x]  
  ask turtles [set x random-float 1.0]
  ask turtle 0 [show x - xlag]
end

I hope this helps . As per Bryan's answer, there is no theoretical restriction on the number of turtles in a single patch, although your computer will have a limit- the more turtles in your model (on any patch) the more memory your model will use. So the short answer is, as far as I know, there is no way to just say to Netlogo, "Spawn infinite turtles on this patch."
If, however, by infinite you really just want enough turtles that you won't run out of them for specific interactions, you could probably get by either by just spawning a large number on that patch or by just sprouting more as needed (my preference).

to setup
  ca
  reset-ticks
  ask patch 0 0 [
    sprout 10000
  ]
  ask patch 0 0 [
    print count turtles-here
  ]
end

to setup
  ca
  reset-ticks
  infinite-sprout
  source-sprout

end

to go

  ask turtles with [ color = red ] [
    fd 0.5
    if any? ( turtles-on patch-ahead 1 ) with [ color = blue ] [
      create-link-with one-of turtles-on patch-ahead 1 [
        tie
      ]        
      set color green        
    ]
  ]
  ask turtles with [color = green] [
    move-to patch-right-and-ahead 90 1
    if pycor = max-pycor [
      ask link-neighbors [ 
        die
      ]
      die
    ]
  ]

  infinite-sprout
  source-sprout
  tick

end


to source-sprout
  ask patch max-pxcor 0 [
    if not any? turtles-here and random 3 = 1 [
      sprout 1 [
        set shape "arrow"
        set color red
        set heading 270
      ]
    ]
  ]
end

to infinite-sprout
  ask patch 0 0 [
    if count turtles-here < 1000 [
      sprout ( 1000 - count turtles-here) [
        set shape "circle"
        set color blue
      ]
    ]
  ]
end