If `raw_documents` is iterable, then we either need to realise it as a list, or just choose a number that more-or-less benchmarks well, although of course this depends on both the average document length and its variance. Ultimately, I don't think there's going to be a one-size-fits-all solution for this problem, but `10` seems too small. It might be better to let the user decide, although adding new parameters is undesirable and it would be bad to encourage a user to spend vast time calibrating such things unless really necessary...