This is the 951st most frequent Russian word.
семи
"Семи" is the dative or prepositional plural form of "семь," meaning "seven" in English.
In this sentence, 'семь' (not 'семи') is used as a numeral meaning 'seven', but there is no direct occurrence of 'семи'.
Семь дней в неделю работают магазины.
Shops are open seven days a week.
Here, 'семи' is used as a dative plural numeral modifying 'зрителям' (viewers).
Этот фильм понравился всем семи зрителям.
All seven viewers enjoyed this movie.
In this sentence, 'семи' is used as a dative plural numeral to indicate to which trees he approached.