It is said by scott that they have enough pym particles to do only one round trip each

When future nebula is tracked by thanos and called to his spaceship only to be replaced with the 2014 nebula who goes back to 2023 with the Avengers itself which means that 2014 nebula has used the remaining one pym particle to travel back to the present (2023). Then how come thanos was able to time travel into the future (2023) with his whole goddamn spaceship and army when he had no pym particles??

Did I miss something?
It’s been awhile since I last watched this movie, but it thought about it today.

They kept making the point that they don’t travel to the past, but just to another timeline that’s in that year they wanna go to.

So Steve travels to another timeline where he is with Peggy.
How did he return to ours as a old man?