The Embassy of Finland gave me a visa valid from 10.10.2018 to 29.11.2018, but the number of days is 50. When will I have to leave Finland?
Schengen visa have both a duration of stay and a validity period. The duration is the maximum number of days you can visit (the first and last day count full) and the validity is the period when you can take those days.
Usually the length of the validity period is longer than the duration of stay. That means you can shift your travel dates a bit, e.g. if a business meeting gets rescheduled, but you cannot extend your travel beyond the duration.