Under Quotes, then Settings, then General, the Quote numbering section decides what your quote numbers look like.
A live Next number preview shows what the next quote will be called, so you can see the format before saving.
A quote keeps the number it was issued, even if you change these settings later. Old quotes never renumber, so a number you emailed a customer still matches.
Once quotes exist you cannot lower it, because that would hand out numbers already in use.