I have a Qitari essence and two Frosty Jewels. When I go to add my supplies, it only lets me pick one gene slot to place a jewel in. Can I only use one frosty jewel per sculpt? I don't see anything in the fest overview about it, nor does it say in the item tooltip that I can only use one at a time, so I'm at a loss.

It depends on what ice water you use-hoever over each one with your mouse to check.

Ah, so I'll need an ice water to use more than one? Thanks for the info!

Specificly a higher-tier ice water (not a regular ice water, since those give only 1vis) but otherwise, yes. (Although check what essence and ice water you use-they all have different maximums). There are 3 different kinds....so yeah ;)

To further clarify what Orca said, it does indeed depend on the water AND the type of cata you have. Some examples...

Regular (blue) or fabled (purple) cata + lesser water: 1 visible, 1 jewel can be used.
Regular or fabled cata + greater water: 2 visible, 2 jewels can be used.
Regular or fabled cata + exceptional water: 3 visible, 3 jewels can be used.

Majestic cata (orange) + lesser water: 2 carries, no jewels can be used.
Majestic cata + greater water: 1 visible, 1 jewel can be used.
Majestic cata + exceptional water: 2 visible, 2 jewels can be used.