That depends on how much you want to spend, how handy you are, and how much work you want to do maintaining it. I put in a 3' x 6' pond, with filter, waterfall and currently 10 goldfish (6-8) now. There are self contained, ready to go units that you jut plug in and fill with water, or even solar that only work when sunny. Amazon has a lot of them, but there are also specialty web sites.
https://www.amazon.com/s?k=waterfoun...l_2zbhkbmg2j_b
https://www.thepondguy.com/category/...SAAEgIOBvD_BwE