I can't find a recommeded software version for the qfx5120.
It's not listed in https://kb.juniper.net/InfoCenter/index?page=content&id=KB21476
Am I looking at the wrong place or why is there no recommended software version for this switch?
The reason is primarily due to the platform being so new that JTAC does not have enough experience to recommend a specific release.
If you use the QFX5120 for EVPN-VXLAN related tasks I would go for 18.4R1-S1 as a lot of EVPN related features has been added in 18.4R1 (https://www.juniper.net/documentation/en_US/junos/information-products/topic-collections/release-notes/18.4/jd0e12462.html#rn-junos-qfx-new-and-changed-features).
Otherwise 18.3R1-S2 should be sufficient.