I sow 3 types of basils around late february by the south facing windowsill. Now they are 12 to 20 cm tall (and I have been pinching them down twice). They do need more light and warm and respond very well to some light liquid fertilizer.
The Thai basil is the tricky one, it do nothing for weeks yet other type has romp away, but now he is cathing up too and growing to the similar height. Give it some light feed and move it somewhere sunnier and don't overwatered them.
Good luck.

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