user: ### NOW ANSWER:
Question: What is the number of participating parties when registered voters totaled 643629?
Columns: ['Electoral District', 'Registered voters', 'Seats in Congress', 'Candidates per party', 'Participating parties', 'Total candidates']
Types: ['text', 'real', 'real', 'real', 'real', 'real']
Sample row: ['Amazonas', 179331.0, 2.0, 3.0, 17.0, 47.0]
SQL:

assistant: SELECT COUNT("Participating parties") FROM "Table" WHERE "Registered voters" = 643629.0;