<$setvars _r="[prefix[mytiddler]random[1]]" random="\{{\ [r] \}}\">
<<random>>
</$setvars>
random="\{{\ [r] \}}\") is put as (random="\{{{\ [r] \}}}\") in Tobias example. I had to make this change to your code to get multiple tiddlers to show up.
If I understand the documentation correctly, you will need to save and reload the TW in order to have the results change. When I made 3 tiddlers and saved, I was lucky enough to get one with spaces on the first reload.
If you're impatient, take out the tiddler with no spaces to force it to show that spaces will work. Or make the count 3 (this only worked after fixing your code to use 3 curly brackets).
Good luck,
Mark