How many 147 were made in a decider frame

This information can be obtained from the maximum break page on Wikipedia. The section "match-winning maximum breaks" lists 18 breaks in total and 5 of them are in deciding frames.

Here are the 147s made in deciding frames:

• Stephen Hendry - 1997 Charity Challenge, Final
• Ronnie O'Sullivan - 2007 UK Championship, Semi-final
• Mark Davis - 2017 Championship League, Group final
• Mark Davis - 2017 Championship League, Winners' Group
• Martin Gould - 2018 Championship League, Group 6

In the Championship League, almost 200 matches are played and each match is won by the best of 5 frames. Therefore it is very probable to see a 147 in a deciding frame.

We should also note that Ronnie O'Sullivan is the only player who has such a break in a ranking tournament. On top of that it's a UK semi-final.