Great if you are starting from your own seed... but not so from commercially grown produce styled seedlings in cramped pots.... You are better off to seed sow for your self, say early February unless you have ideal bright light and warm temperatures found in a hot house...then go crazy! You may try to separate the clump, but they may perish... I love fresh basil.... my freezer is set for the pesto filled wintery/spring days still ahead...
YES, I would recommend separating the basil for two reasons: you get way more basil this way & greatly reduce the risk of mould growing because of improved air circulation. Watch video: Discover This One Weird Trick to Grow Ten Times More Basil! | 321 Garden