how many displays can I connect to a Mac mini (late 2018)?

I used to connect 3 with that mini 2018. Of course, you have only one hdmi port, but with 4 TB/usb-c ports (more generous than newer models) and adapters you will safely connect 3 and yet be able to extend displays.

i now use 2 for some other reason

See Mac mini (2018) - Technical Specifications - Apple Support

The 2018 Mac mini can take three displays.

One display on Bus 1 One display on Bus 0 One display on the HDMI