I think the stock saddle is 144 mm, but I could be wrong.
Since the shop where you bought the bike is a Trek dealer, they will have a measuring device that you can sit on to find out if the saddle you have is the correct width.
What saddle is on the MTB?
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles