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There is an exact cap. It's not the # of email addresses / # of lines of the filter.
It's the number of characters in the entire textarea (across all lines, including line break characters): 50,000 characters.
Note that some browsers count a line break as 2 characters (CRLF) and other browsers count line breaks as one character (LF). This means the real-world limit is actually different depending on browser: in one browser you could add 2,380 20-character email addresses, while another would only let you enter the first 2,272 of those addresses. (Once the Smart List is saved, it'll work regardless of the browser used to create it.) Strange but true.