How many matches have there been at the top level which have ended up with a tied score and a winner? In the current FIDE Grand Prix tournament in Jerusalem the first round match between Pentala Harikrishna and Sergey Karjakin ended in a tied score, 4.5-4.5, but Karjakin won because the last game was an armageddon game with him as black.

In the 1950's Botvinnik "won" two world championship matches as defending champion with a score of 12-12, one in 1951 against Bronstein and again in 1954 against Smyslov. At the time there were no tie-breaks for world championship matches. Instead the defending champion retained the title.

That makes 3 matches. Are there any more?
As @Remellion pointed out, the 1987 Kasparov - Karpov world championship match also finished in a tie, 12-12, with Kasparov retaining the title.

That chessgames.com link leads on to the 1910 Lasker vs Schlechter world championship match that finished 5-5 with Lasker retaining the title. The interesting story of the match is told here. With one game to play Schlechter was leading 5-4 and only needed a draw to become world champion. Instead of playing safe he played for the win but after a game lasting 11 hours spread over 3 days he lost and Lasker retained the title.

Another tied game which had a winner was the 1983 Smyslov - Hübner Candidates Quarterfinal Match which ended 5-5 and then was extended with 4 full length tie-break games (all draws) for a final score of 7-7. 

The match was held in Velden, Austria, about 500 meters from the Casino Velden and so to decide the winner they used the roulette wheel. If the ball fell into a black number Hübner would be the winner and if red then Smyslov. The first spin saw the ball end up in zero (no colour) so a second spin was needed which landed in red, so Smyslov won the match. 

Robert Byrne, reporting in the New York Times, thought this was very unfair and proposed that more rapid games should be used to break ties when the tie-break games at the regular time control failed to produce a winner. Perhaps this was the first time rapid games had been suggested as a tie-break mechanism?

So, that takes the total up to 6.