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Hi Belen, I think having one brand page is enough. You don't need to have 15 Google+ pages and I don't think having more than 1 has an impact on SEO or backlinking. You can use this G+ page to promote content and post updates like any other social media account, and link to it to your website's header/footer. Apart from what I've mentioned, I think there's really no other use for G+, unfortunately.