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This depends on which Google Network you want to run remarketing ads.
Display is the most lenient. For Google Display Network your remarketing audience must include a minimum of 100 cookies (basically users, but it roughly equates to browsers). You can build these pretty quickly as many businesses will have at least this many website visitors every day.
For Google Search Network and YouTube your list must include a minimum of 1000 cookies.
If you want to increase the number of cookies, choose a longer timeframe for cookie collection. This is a good practice anyway, so that you don’t lose these long-term visitors from your list. You can set your remarketing list to collect users up to 540 days since they last interacted with your business, but by default it is set to only 30 days.
If you want to restrict your remarketing audience to people who have added products to cart or interacted on your website you can definitely do so, but keep in mind that the more restrictive you make the requirement, the smaller your list will be and you might not meet the minimum size requirements to use the list straight away.
This tip and many others are found at https://www.petramanos.com/category/tips/