A remarketing list must have at least 100 users on it before you can show them an ad. This not only protects the privacy of those who make up your list, but also helps ensure that your ad will have a big enough audience to make some impact.
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Hi there LuckyRage,
Thanks for your question, there have been some great responses here already, which will hopefully help you out.
However, in answer to your question, you do not need a brand new campaign to use remarketing to target your customers, but we would recommend you separate out your remarketing efforts into a new campaign as this will make it easier for you to manage. It will also mean you do not need to worry about your different settings between the Search network and the Display network.
Hope that helps!