Answer by
LeheckaG (1826)
Assuming that the basement has a tiled? ceiling,
then every other or at least every third or fourth tile.
Standard "flood" or "reflector" fixtures fan out about 90 degrees,
or 45 degrees on either side of the fixture.
So about 6 foot or less spacing gives you good light coverage.
"Spot" lighting has a narrower span (about 2-4 feet).