There are thousands of suit combinations in the Encyclopedia of Bridge.
As humans, we can't expect to learn/memorize ALL the odds. But, a knowledge of the basic percentages is important to improving your declarer play.
A finesse is 50-50. (I know, not for you -- because you are unlucky). Unless there is something in the bidding to tell you otherwise, assume you will win a finesse half the time.
When missing 5 cards in a suit they will likely split 3-2
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