Garlic substitution beef stew

The purpose of garlic seems to be add some garlic flavor in your stew sauce. So I think without garlic cloves should be OK, or adding minced garlic or garlic powder is also fine(but caution on the amount not to let garlic flavor too overwhelming.).
Or you can substitute with shallot is also fine if you have it, their flavors are similar in some ways.

All depends on the strength and freshness of the garlic you have to hand. The minced garlic will probably have added vinegar or other additives that may affect your final dish, so I would aim for the powder which is closer to fresh garlic.

As to the quantity, 2 cloves of garlic simmered for 90 minutes will be enough to break down the flesh of the garlic quite considerably and add quite a lot of flavour, although not overpoweringly so. If your powdered garlic is quite potent (try sprinkling some on a freshly cooked french fry to test), I'd use 1/4-1/2 tsp, maybe upping it to 1/2-1 tsp if it is not quite so fresh or if the quantity of meat is a larger portion, say for 4.

When you add it is important as well, the powder will lose some of its flavouring the longer it is cooked. If you want a more dominant garlic flavour, add it towards the end of cooking. Some garlic powder can be rather bitter, and needs cooking out, so I would give it at least 15 minutes in the stew if that is the case. The Schwartz coarsely powdered garlic I use can be added right at the very end as it is not bitter, whereas the finely powdered variety in the tin (e.g. Rajah) definitely needs cooking.