Professional quality blank 310gsm black core cardstock that is used by modern commercial TCGs in industrial offset printers.

A4 (8.5*11in) sheets. Fits in standard printers. A rear-fed laser printer is the most compatible printing technology. It will likely jam any non-rear-fed printer. NOT COMPATIBLE WITH NON-UV INKJET PRINTERS.

Back fed laser printers (check the paper thickness gsm specification of your printer, though it is usually understated) produce good results. The melted toner acts as a sort of varnish (water resistant, contributes some snap) which is on commercially produced cards. For higher quality, you will need to tweak the settings of your printer and color management.

310gsm is closest to Pokemon cards. (.3mil thickness)
(closer than 300gsm, 320gsm, or 330gsm)

Smooth Silk Finish (matte with a slight shine). NOT embossed or linen finish.

The "actual results" photo is from a consumer laser printer after optimizing the settings.
Coated surface: Consumer inkjet ink will not absorb (generally). Laser or UV (flatbed DTF, UV inkjet) are the only endorsed printed technologies. Use other printing technologies at your own risk. You may need to adjust your printer settings.

Each A4 sheet fits 9 poker (63*88mm) MTG/PKM) sized cards.

For professional industry quality, a flatbed DTF printer is best because it uses UV ink and applies a varnish layer. There are combo flatbed/roll DTF printers that can also be used for hot stamping/foiling (very expensive). Great quality results can still be obtained from laser printers (standard consumer).