Can I calibrate more than one hygrometer at a time in the Boveda kit?

As long as there's about a finger's width of clearance around each one,  you can calibrate up to four (4) hydrometers/sensors at once in the Boveda Calibration Kit. 

We recommend calibrating each hygrometer separately—the kit is reusable. (Once it's opened, the bag's preloaded pack is good for up to three (3) months.)

• Measure your hygrometer to make sure it will fit in the kit. The non-gusseted bag measures 5.5"/13.97 cm x 7"/17.78 cm.

• If your sensor is too big for the kit, you can calibrate your hygrometer with a Boveda 75% RH packet in a zip-to-close freezer bag or an airtight plastic container. Just follow these direction on how to calibrate a hygrometer with Boveda.