It is only one to put the belt on if you put the belt Wrong you can’t even get it on the motor pulley.
Now I am confused
917 274822 was made by AYP ( Husqvarna ) so your deck should have 2 spindles and 2 idler pulleys
When mounted it should look like a T so the belt goes from the engine to the idlers then go left or right to the spindle pulleys and directly from spindle to spindle at the back.
The left side pulley should get pulled back when you engage the blades to tighten the belt & release the blade brake.
One or both of he idlers will have a single or double belt keeper to stop the belt coming off when slack
The belt has to go in the V of the pulley with the keeper behind the back of the belt.
Very common for owners to run the belt around the outside of one or more keeper which makes the belt too short & destroys it quickly should you try to mow .
If your deck has only 1 idler pulley then it was made by MTD and that model number is wrong.
On the MTD deck, the entire deck moves down & back to tighten the belt & release the spindle brakes .
Which system do you have ?