I believe that as the 50th captain crosses the horizon, they'll see 49 starships in front of them and 50 starships behind them... surprisingly unsurprising!
However, all 49 starships in front of them will also be crossing the horizon. That is, the light of these 49 previous ships will be moving tangent to the horizon, and the 50th captain will see that light the moment they cross the horizon.
(2/n)