I am lacking room as a gardener, so I've taken up the belief that if it has room for it's roots, then it has enough room. :D As long as it's getting decent sun to it's leaves.... well, it's got room. My poor plants! :D

A full-grown basil may have a spread of about a metre or more (mine are about 1.5 metres wide). That's in a nice hot climate of course. In cooler areas, plants rarely reach optimum size. Take it from there. Basil isn't difficult to transplant if necessary. If grown closer together, just feed more! Basil is a greedy plant.

The radish seed plate in a Planet planter provides a very dense row. I found that it works well if growing basil to cut bunches for a farmers' market. I plant two rows only 5-6" apart which allows me to tack a drip tape between the rows. I'll grant that the row will grow in thick but weeding thins it out a little and I can cut a bunch from a short section of the row.

Usually the biweekly tipping or cutting allows plenty of light and air to reach the base of the plants although not all plants recover. Sometimes when gathering tips I uproot a plant which I never bother to replant.

A wholesaler who buys basil during our growing season referred to Israel Basil which he buys year round. He said that it is pulled and bunched with the roots attached. Just one other way to harvest and thin at the same time.