From what I can tell, and I might be well off here, if you sign a player that fails to get a work permit but given an ESC place, if you play them for the necessary amount of games during the 1 year ESC period, then when you apply for a work permit, there is a higher chance they will get the work permit and free up that ESC place. Had a player from Jordan come into my Harrogate team and he had to have an ESC, but after a year he got the work permit. Did help he was also getting picked for the Jordanian national team, but they are not a top 50 nation so don't think that helped much.
Also, the higher the league, the more ESC places you get. Sure I had 2 in League 2 and now have 4 in the Championship.