If in the parlay mode bet, 3 or 4 selected matches are more than enough I think. but if you refer to the Op bet with 8,10,15 matches. I can't imagine it. Just imagine, just predicting one match is quite difficult and the results are not always accurate. especially if you choose 8,10,15 from the slip list. for sure, the reward is quite large. so, the best option is to cash-out if it's only 1 game left. except, you are confident that you will win all the matches.
To be honest, i never take a cash-out. because usually, I only bet for 3 or at most 4 games. that too, after going through a rigorous selection from what I have previously analyzed. in fact, to win 4 matches is not easy at all. so, just imagine if we do as Op said in this thread with 8,10,15 matches. so how likely is it that he will win, for sure the probability is very small. unless he's really lucky.