I count twenty.
15
abscondaccordanceacrossaddressadulteressaggressiveagelessalabasteralasalbatrossamassarisearouseassimilateaspenastuteasaspireassuageassetassuredassortedaspirinassassistassertascendassessaccessasteratlasaudience
U.S. states that end with the letter a:AlabamaAlaskaArizonaCaliforniaFloridaGeorgiaIndianaIowaLouisianaMinnesotaMontanaNebraskaNevadaNorth CarolinaNorth DakotaOklahomaPennsylvaniaSouth CarolinaSouth DakotaVirginiaWest Virginia
South Carolina and South Dakota are U.S. states. They begin with the letter S.
There is no US state that contains all of the letters "RCSCSS". Only two states contain three letter S: Massachusetts, which has no R and only one C. Mississippi, which has no R and no C.
Assessments contains five letter "s". I can't think of another example.
The Last letter to U S is found in United States of America
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dangerous
phases
South Carolina South Dakota
boa constrictors contain the letters trts