This is the 2605th most frequent Turkish word.
sürü
"Flock" or "herd" (a group of animals or people).
Here, 'sürü' is used to denote a group of animals, specifically sheep, emphasizing its meaning as 'flock' or 'herd'.
Bir koyun sürüsü otlakta geziyor.
A flock of sheep is grazing in the pasture.
In this sentence, 'sürü' is used colloquially to mean 'a lot' or 'a number of'. It implies abundance in a less formal context.
Penceremin önünde bir sürü kuş vardı.
There were many birds in front of my window.
Here, 'sürü' conveys the idea of a large quantity, metaphorically referring to 'a lot of' time being dedicated to an activity.