Depends how big your plants are. Let's say each lavender plant has a 3 inch diameter. Then you can get 4 plants evenly spaced with 1 inch between each. They will quickly spread out, so you may need to divide, or you can start with only 3 plants.

welcome to GoY :o) where in the world are you by the way?

as Bathgate say's depends on how big the plants are. lavenders are happy in poor stony soils [Mediterranean slopes] so they will be ok in a 10" depth. but I would go for 2 plants in each trough. they will be happier in full sun but the pots may well dry out too quickly.

If you're in the UK or anywhere its not yet warm, and those plants were in a greenhouse or under shelter when you bought them, you will need to harden them off before planting outdoors. Do that by standing them outside in the middle of a day for around 3 hours, then bring them under cover, leave them out the next day and so on for longer time periods until, on a mild night, they've been out all night. Depending on temperatures outdoors, hardening off will take about a week, longer if the weather becomes cold again, and especially if your plants are already in flower. Average spread for Lavender stoechas is around 30 cm or 12, maybe 15 inches, depending on the variety, so allow that much space for each plant in your planting box.

Hi Anna! Your question is well answered, I just wanted to say Hi and Welcome to Goy! I hope you will love growing your plants on your patio. :)