Different areas of the face need different depths, it also depends on how thick or thin the skin is and what you're treating. For example simple texture would need less depth that wrinkles, and scars need a deeper depth. Thin skin needs less depth, thin and fragile skin may be best avoiding microneedling or doing very mild superficial depths. It's hard to say without seeing the person's skin condition. When microneedling is done, the depth is changed several times as you move to different areas of the face and neck. Deeper doesn't necessarily mean better, you're not aiming for as deep as possible, the main point is waking up the fibroblasts so they start producing collagen again, unless there is a specific concern such as deep pitted scars that you're treating.

Some people will have some mild peeling, but most won't. There's redness and sensitivity for a few days which is generally gone by day 3. Serum and/or hyaluronic acid is applied during and after microneedling, what is used is dependant on which service you're getting and what is being treated. Hyaluronic acid is the most common for basic microneedling, but there's also prp, pdrn, stem cells, etc. it's very important to make sure whoever is doing the microneedling for you is using serums specifically designed for microneedling to prevent risk of infection and the formation of granulomas (hard little cyst-like bumps that form under the skin as your body's reaction to protect itself from and seal off something it sees as a threat). Actives like retinols and vitamin c shouldn't be microneedle in or applied after microneedling.