Databunch and iterations

I am not able to understand why when creating a databunch, some times the progress bar appears with 1 iteration to be completed
[i.e., 0.00% [0/1 00:00<00:00]
and some other times it appears with more iterations to be completed
[i.e., 0.00% [0/8 00:00<00:00]
What triggers this behavior? Is it maybe the size of data I am using to create the databunch each time?
Usually when I use more data the number of iterations increases. But what is happening in these extra iterations that it is not happening when I have less data and only one iteration appears?

Thank you