This can be modified using custom difficulty settings. In total, there are currently 7 unique cursed possessions in the game, and with standard difficulties (EXCEPT for Insanity), 1 of them will spawn on the map every match.

I also created a guide covering all the equipment types, uses and tiers added in the Ascension Update overhaul, so if you're curious about the new systems or want to learn the basics, check it out. Quickly, if you are also looking for tips to correctly identify the ghost type, I also created a guide to hopefully help with that as well, linked below. If something in this guide is found to be inaccurate or outdated, I apologize, and encourage posting corrections and updates in the comments below. This guide pertains to Phasmophobia v.0.8.1.3, and may become inaccurate in future versions.