The Garlic: Roasted garlic is much mellower than raw garlic so don't let the idea of multiple cloves intimidate you. If you choose to work with one head of garlic, I still strongly encourage you to roast at least two (or three) heads because there are so many ways you can enjoy this delicious, caramelized garlic paste (which stores well in the fridge): as a spread on bread/crackers, in dips/hummus, in salad dressings, as a spread in sandwiches or as I did this past weekend, smeared all over our black bean burgers - so delicious!
The Flour*: All purpose flour creates a smooth and delectable Béchamel however I did try tapioca starch/flour (a gluten-free substitute) in one version of this soup - it worked perfectly well in terms of thickening the soup although there was a slight difference in terms of smoothness - a very mild graininess that is not detectable in AP flour. I still found it very palatable but if you go with tapioca flour my suggestion would be to whisk it vigorously to create the smoothest texture possible.
Garnish Options - I enjoy this soup topped with some black pepper or paprika, seared mushrooms and fresh thyme for garnish but my boys also love it with cheese. You can play around with this element to suit your tastes.
The Cream - Half and half is what I generally use as my 'heavy cream' - I find it adaptive and perfectly luxuriously but you can sub heavier cream in this portion of the recipe as preferred.
The Mushrooms - Mushrooms are about 90% water by nature so you want to be careful not to rinse or soak mushrooms to avoid ending up with a soggy mess. Instead, use a brush or cloth with a little bit of water to clean. As for type of mushroom, I went with a common mushroom here but you could sub any variety of choice. Oyster mushrooms, for example, have a delicious resilient, somewhat chewy, texture that is irresistible in soup. Depending on where you live and what you have access to, seasonal varieties might also come into play.
Soup Consistency: This is a creamy béchamel (somewhat thick) soup like potage -- you can thin it by adding more broth, little by little, to desired consistency. If the soup ends up not as thick as you would like, simply add a little tapioca or cornstarch slurry at the very end (in a small bowl, whisk 1 flat tbsp of tapioca flour (or cornstarch) with 1 tbsp of water and then add the slurry to the soup, to thicken somewhat.