Conversation so far:
who becomes president if vice president can't
If the vice president is unable, the speaker of the House of Representatives is next in the United States presidential line of succession.
who is next after speaker of the house in the United States presidential line of succession
After speaker of the House comes president pro tempore of the Senate.

Question: how many times has the vice become president
Self-contained rewrite: how many times has the vice president become president