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4-6 times is good limit but in modern SEO tactics its good to have exact keyword in Meta Title and description then go for one exact match keyword in website content and uses the keyword variations like if its buy Magento extension then user it in Metas and when it comes to website content use its variations like module, plugin, addons etc.

You don't have to enter the allow commands. By default it is allowed for Google to crawl and index whole website. You only need to mention the disallow code to stop crawlers from indexing unwanted URLs like customer accounts, admin urls etc.