Single site is enough for promotion purpose rather than different sites. Sub domains are good but for all purpose these are not good. Create folders and place your specific information over there. Your site will convert in Authority site which will give more
weightage to your website.

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both things work fine but i suggest folders work great in comparison with sub domains.

Also it depends what is your business models some times sub doamins also work good as google has all sub doamins, if you have more content in same niche then a big site is good and if you have various business models then multiple sites are work fine as those sites are easy to maintain.

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