The X-Carve Pro spindle uses an ER-16A collet type. The thread size and pitch are M22 x 1.5.
It can accomodate 1/8", 1/4" and 3/8" collets. You can purchase them here:
If you are new to ER Nuts and Collets, please see the attachment with instructions on how to swap them.
Alternatively, you could purchase additional collet nuts to have it ready and avoid swapping.
From time to time, you will need to inspect the collet and collet nut to ensure there is no debris lodged and preventing the bit from being fully secured. A loose bit can break or cause runout.