2 screws in 2.5" SSD unless it's a weird adapter\tray then maybe 3 or 4.
HDD all 4, all the time. They physically vibrate and move and that vibration imho, should be controlled\managed as well as possible. Maybe that's beyond what matters, but I make sure my spinners are mounted "solid" or "snug" if using a foam damper\separatater\spacer in the tray.