user: Question: How many times is the formula Tl 2 Ba 2 CuO 6?
Columns: ['Formula', 'Notation', 'T c (K)', 'No. of Cu-O planes in unit cell', 'Crystal structure']
Types: ['text', 'text', 'real', 'real', 'text']
Sample row: ['YBa 2 Cu 3 O 7', '123', 92.0, 2.0, 'Orthorhombic']
SQL:

assistant: SELECT COUNT("No. of Cu-O planes in unit cell") FROM "Table" WHERE "Formula" = 'Tl 2 Ba 2 CuO 6';