A text collection should give you a good overview of a given topic. Depending on the domain covered, this can be smaller or larger. It is possible to imagine domains with very few texts, which are nevertheless exhaustive and self-contained (eg "North wind", "East wind", "South wind", "West wind"), but it is also possible to imagine collections with a much larger amounts of texts.
At the same time, one can also have collections of very short texts (eg lullabies) or very long texts (epics). So, there is no hard and fast quantitative specification of how many words/sentences/texts a text collection should contain.