user: Question: When Vista Radio is the owner how many call signs are there?
Columns: ['Frequency', 'Call sign', 'Branding', 'Format', 'Owner', 'Language/Rebroadcast']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['FM 93.7', 'CJLT-FM', '93.7 Praise FM', 'Contemporary Christian music', 'Vista Radio', 'English']
SQL:

assistant: SELECT COUNT("Call sign") FROM "Table" WHERE "Owner" = 'Vista Radio';