The recipe calls for the onion/shallot to be sauteed with garlic, but there is no garlic in the ingredients list. What is the suggested quantity?...

It's personal preference more than anything else. The recipe isn't going to fail if one omits the garlic.

Based on decades of eating dishes that contain garlic, I'd go with one or two garlic cloves because I am personally not a fan of protruding garlic overtones. Some people love super duper over-the-top garlicky dishes but I do not count myself amongst them.

Anyhow do as you wish. After all you and your dinner guests are the ones who are going to eat it not any of us here.

Best of luck.