The number of coats and quality will vary depending on the application. For example, when applied as wall paint or with wood finishing products like varnish, more layers are needed for better durability than with a floor care product like polyurethane.
Oil-based polyurethane will usually require less coats than water-based polyurethane due to it having a higher level of resistance to abrasion. Nevertheless, the number and quality will also depend on how much polyurethane you apply for each coat as well; this can be determined by your needs which may range from durability or maintenance depending on what application it’s being used with ( flooring vs furniture).
Water-based polyurethane is much thinner than oil-based. The number of coats you may need vary between products, but usually four to five coats are best.
After about 24 hours of drying time, sand the surface until it is completely smooth. Unless the surface is completely dry, it’s best not to apply a fresh coat of polyurethane. Wait for 12 hours before doing so.
Usually, with oil based polyurethane you should apply two coats at least because one layer does not provide enough protection against abrasion.