As Chris says, what you describe is referred to as the unsound practice of data peeking.
This procedure will inflate your type I error rate. So while you correct in each step for FWE and your significance level is at alpha=0.05 every time, the final familywise error rate of your analysis is a lot higher than 0.05!
My suggestion:
(1) use only the 10 first subjects
(2) perform a group analyses
(3) upload the resulting statistical map to https://neuropower.shinyapps.io/neuropower, and follow the instructions. Now you’ll now how many subjects you’ll need for a high-powered study
(4) Discard these 10 subjects and use only the subsequent subjects for your final analysis !! This seems hard, but it is important that the sample on which you based the power analyses are independent from the final sample, otherwise this will again inflate your type I error rate.
Joke
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