Recently I bought an imac mini 2020, wich has 4 thunderbolt ports, 2 USB , and one HDMI. I want to use my old iMac late 2009 which has one mini displayport, as a external monitor,(Target Display Mode). Could you please tell me which cable I need to c

I assume you have the 2018 Mac mini. It's the latest model. (The 2020 iMac has two Thunderbolt ports and no HDMI.)

If the 2009 iMac is 27" it should work. You will need a USB-C to Mini DisplayPort cable. A Thunderbolt adapter won't work.

https://www.startech.com/AV/display-and-video-adapters/usb-c-video-adapters/usb-c-mini-displayport-cable~CDP2MDPMM1MB

If other Mac is a Mac mini, it's the 2018 model even if purchased in 2020, so shouldn't be affected by that restriction.

Use your iMac as a display with Target Display Mode - Apple Support

Target Display Mode 2009 & 2010 iMacs

Per Apple, this won’t work:

You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display.

Your “other” Mac is too new.