Substitute celery seed for fresh celery in soup
If it's one stick in a pot serving several people (as it sounds from your question), it's not a major part of the flavour.  The seed also has a different flavour to the sticks and can be quite strong*, so I'd be sparing and use a pinch, crushed.  Serious eats also talks in terms of pinches, though they think it tastes "exactly like celery" or "very similar". 

If there's quite a lot in there, the substitution probably isn't a good idea as the differece in flavour will be more apparent and it will also affect the texture.

*The last time I used celery seed it was home-grown which might taste stronger than bought.