What is the minimum interaction per item needed for Amazon Personalize SIMS recipe?

You can tune what has relevance - so you can specify the minimum number of user co-interactions between two items (defaults to 3), have more or less influence to long or short user histories (ignore me as I buy 2 things a year, or ignore you as buy 1000 per day), and also tune the min/max percentiles where you want to include or ignore item interaction counts.

So as the co-interaction value defaults to 3, that’s the default, but tune it if you want. You could make it so that an item with just 2 people co-interacting with it once is included, but normally the percentile settings would rule those out.