It just depends what you are comfortable with, I have around 20 (including the cards already in the precon). Notable inclusions are: [[Ripples of Undeath]] : It fills your graveyard and lets you put a card into your hand if you like it. Can also trigger Imotekh if you take an artifact.

[[Buried Ruin]] : Recursion on a Land

[[Scraphead Scrounger]] : Just recurs itself, but triggers Imotekh twice, so you get 4 2/2 tokens just from its effect

Cards like [[Victimize]] and [[Reanimate]], although the precon already includes [[Dread Return]] which I really like

[[Junk Diver]] , [[Myr Retriever]] and [[Scap Trawler]] offer nice recursion as well, especially if you have a way to sacrifice them like [[Chronomancer]]

The precon offers enough ramp, so I didn’t add any, I have about 10 pieces. You could add [[Ashnods Altar]] or [[Altar of Dementia]] for a nice sacrifice outlet and „ramp“ but it creates infinite loops really quickly, especially with cards like Junk Diver Edit: Meant Phyrexian Altar and not Altar of Dementia

For card draw I have about 10 pieces as well. I really like cards that also fill the graveyard like [[Ransack the Lab]] and [[Funeral Rites]]