About 4 inches deep is fine. Plant successive groups of seeds. They can be planted quite close togther as you are harvesting the tender young leaves. On a deck, if its strong sunlight, the cilantro will not like that. Cilantro prefers cooler weather and mostly shade. I start it in 2"x2" pots and plant maybe 10-20 seeds in each pot. If I want seeds (coriander), I allow it to bolt and continue into flowering, then seed formation. There are a few coriander varieties that are slower to bolt. But usually once those nice parsely shaped leaves go, the plants send up thnner fern like leaves, which can also be harvested. After that, avoid the stems as they do get woody and tough.