If the winner of the Challenge Cup gets a spot, it'll mean that the top 7 sides after us or Saracens qualify for the Champions Cup, so effectively if Saracens win it, it'll be the top 8 sides who qualify and the same if we finish in the top 8. If we finish 9th, then the 8th place side will miss out.
My understanding is that the 8 URC sides will be the top side from Ireland, Wales, South Africa, Scotland or Italy and then the next four sides in the league table who haven't already qualified. At the moment that's Leinster, Ospreys, Sharks and Glasgow as the top ranked sides from their countries and then Munster, Stormers, Ulster and the Bulls.
It may be that there are two South African sides in the Challenge Cup. With the Premiership remaining 13 teams, there are 5 Premiership, 8 URC and 6 Top 14 sides, a total of 19 teams. The rumour is that the Cheetahs will be offered a spot to make it a 20 team competition until the Premiership goes up to 14 teams.