user: Question: How many different riders are there that won riding OMR Tsunami?
Columns: ['Year', 'Location', 'Distance (miles)', 'Rider Names', 'Horse Name', 'Best-Conditioned Horse']
Types: ['real', 'text', 'real', 'text', 'text', 'text']
Sample row: [2012.0, 'Bandit Springs, OR', 33.0, 'Ben Volk, Dennis Summers (M/M)', 'OMR Tsunami', 'Magic Sirocco']
SQL:

assistant: SELECT COUNT("Rider Names") FROM "Table" WHERE "Horse Name" = 'OMR Tsunami';