Hi,
You might see relevant part numbers in the data sheet: https://cloud.kapostcontent.net/pub/6f346f0e-30e3-452c-86cd-56795f6a1a65/x450-g2-data-sheet.pdf
10311/10312/10313/10323 - QSFP+ passive copper cables of length 0.5/1/3/5 meters,
10336/10337/10315/10316/10318 - QSFP+ active optical cables of length 3/5/10/20/100 meters.
For longer distances you might need QSFP+ modules and a patchcord.
Regarding stacking, here you might see the general procedure:
https://gtacknowledge.extremenetworks.com/articles/How_To/How-to-create-stack-with-Summit-X460-48p-and-Summit-X460-24p
https://gtacknowledge.extremenetworks.com/articles/How_To/Getting-started-with-Summit-Stacking
Hope that helps,
Tomasz