How many preserved lemons to use?

...And hoping you're adding a little olive oil to help distribute things. Love the cumin and thyme.

Yes, if they are traditionally preserved lemons, they're very salty. You wouldn't want to add more salt until you taste your diced preserved lemons mixed. in. To be safe, mix 1/2 of a preserved lemon, diced (pith and peel) into 1/2 a pan of your root vegetables. See how that tastes. If it's too salty, then you just add the remainder of the pan of root vegetables to distribute the lemon. If it's *not* too salty, add a little more diced preserved lemon. Remember that the lemony taste will mellow as things roast in the oven, but the salty taste will not reduce.

I'm roasting a 9x13 pan of carrots, onions, squash, and celery with some cumin and thyme. Would more lemon = more saltiness?

It sort of depends on what root vegetables you're using... some are sweeter than others.

Depending on how lemony you want your root vegetables, I would use 1/8 cup diced preserved lemon (that's including the peel) to 1 cup root vegetables. You can taste that mixture and adjust up or down.