I am a citizen of the USA and I'm planning to be in Brazil for about 5 days on a cruise in November-December 2018. I then plan to do another land tour, which includes Brazil somewhere between January and March of 2019. Will one visa work for both trips?
The answer should be yes. If you have applied for a tourist visa these are given with a validity of 10 years (the old, traditional visas) or 2 years (the new e-visas coming online soon). Both are valid for multiple entries. See here for more information and information on other types of visas.