Do I need a tri-band router or is dual-band enough?

Dual-band (2.4GHz + 5GHz) is fine for typical apartments and homes under ~30 connected devices. Tri-band adds either a second 5GHz radio or a 6GHz band, which matters if you have many simultaneous high-bandwidth clients or want to dedicate a clean band to VR, gaming, or wireless backhaul.