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תפסת מרובה לא תפסת just means that when the Torah says a word in plural form, the most we can infer from it is that it referring to a minimum of two.
The rationale behind it is that even if the truth is more than that, the minimum is still part of it, (see Rashi and Tosafos Rosh hashana 17a). For instance, let us say that we have a contract stating that rent is ‘hundreds’ of dollars, yet we don not know if it means two hundred three hundred or even more. Yet one thing we do know, and that is, the rent is a minimum of two hundred, for even if it was three hundred, part of three is two.
This statement was not said in regards to prayer, although there are different Mamarai Chazal pertaining to it.