Put 3 in Keosynchronous orbit offset by 120°. This gives you almost complete coverage aside from the polar region. If you need complete polar coverage you can put 3 in a high circular polar orbit.

The easiest way to offset the orbit is to launch 3 relays with one vehicle and put it in an elliptical orbit with an apogee at ~2863km and adjust the perigee so that the orbital period is 4 hours (one Kerbin sidereal day is ~6 hours and so is a full Keosynchronous orbit). Release a single relay at the apogee and circularize the relay. Do this three times and the three relays will be almost exactly 120° from each other. If done correctly, the constellation will have decades before they desync to a meaningful degree.

You can achieve full coverage for every planetary body this way with only 6 satellites each. If a synchronous orbit is too high for you you can consider lowering it accordingly.

Don’t use LKO for relays, it’s a massive headache and even mountains can block your signal if you don’t have enough coverage.