A kimono itself is generally only 1-2 layers (depending on if it's lined and if you count a lining as a layer). Beyond that, modern kimono outfits (regardless of formality or social status) generally have the same number of layers: the kimono itself (1-2 as described above), a juban, and a slip that goes under the juban. So that's 3-4 layers depending on if you count a lining as a layer. If you count the obi that could add another layer, but I wouldn't count the obi since its function isn't the same as that of the kimono, juban, or slip. There are ways to fake the look of additional layers, e.g. false layers added into the skirt and/or sleeves or a kimono, or additional false collars added between the juban collar and kimono collar. These give the look of extra layers without the bulk, weight, and heat of actual extra layers. THe one major exception to this that normal, everyday people might ever wear is the wedding ensemble, which includes at least one additional layer (the uchikake). Hair accessories are going to vary pretty drastically depending on the situation and hair style. For everyday wear I would say none or 1 would be most common. For a wedding with a traditional hairstyle, there's going to be an entire multi-piece set that can include 5 or more separate pieces. A maiko can have 6 or 7 separate pieces in her hair for everyday wear when she's young.
Depends on social status, and formality.
