Ten years make a decade
A decade is a span of 10 years. Two decades are 20 years, and half a decade is 5 years.
Convert a longer span
Multiply decades by 10 to get years, or divide years by 10 to get decades. A century is 100 years, so it contains 10 decades.
Be clear about calendar labels
A period such as 2020–2029 is commonly called the 2020s. If you mean an exact anniversary interval, give the start and end dates too; a label alone does not specify the day or month.