Open enrollment season starts Monday for Medicare Advantage plans, and customers will find, on average, about two dozen plans to choose from, according to the Kaiser Family Foundation.

Customers will have until Dec. 31 to review health and prescription drug coverage and join plans or make changes to their current coverage. They used to have a window from Jan. 1 to March 31 for additional changes in some cases, but that will be eliminated starting next year.

Medicare Advantage plans are privately run versions of the government's Medicare program that provides insurance for elderly and disabled people. Subsidized by the government, the plans offer basic Medicare coverage topped with extras like vision, dental or hearing coverage at premiums lower than standard Medicare rates.

Insurers offer hundreds of different plans around the country, all with their own sets of variables like different deductibles, premiums and co-insurance. UnitedHealth Group Inc. and Humana Inc. are the two largest Medicare Advantage plan providers.

This year, about 24 different plans will be offered in each county, even though the overall total fell about 13 percent nationwide, Kaiser said, citing in part new rules encouraging consolidation of low enrollment and duplicative plans.

About 11.8 million Medicare beneficiaries, or nearly a quarter of the total Medicare population, were enrolled in a Medicare Advantage plan as of Sept. 30.

Customers can visit or call 1-800-MEDICARE for more information.