Is there any way of determining the value of k for selecting top k sentences in text summarization
That's probably mostly determined by how large of a summarization you need.  In other words, if the summary must fit into some constraint (e.g., 400 characters or less; at least 50 words) then what's an appropriate setting of k to satisfy the constraints? Relatively speaking, it's similar to hyperparmeter optimization in ML.
Also, the quality will tend to be affected. Too small of k yields results that probably aren't effective. FWIW, I try to use k >= 3 generally.  Too large of k and the results become less readable.
I used the TextRank algorithm for ranking sentences of some articles. The total number of sentences in the articles range from 10 to 71. I wanted to know if there is any way of determining the value of k for selecting the top k ranked sentences as the summary. Or is that fixed to be some number?