Minimum spacing between PCB components
Unless you have

high voltage issues

crosstalk (by electric fields) between components

heat dissipation issues


then pack the components as tightly as you are able to insert the thru holes.
You can even have components flush with the PCB, with alternate components raised up a few millimeters, with the solder pads offset as you wish.
And you can hand_form the leads, to be LONGER than usual, to act as very useful jumpers across troublesome traces.
In other words, use the components as a way to have 3 layers or even 4 layers of routing.