50 Lines should not cause an issue.
Are you creating the PO from standard po forms or the buyer work center or PDOI?
What issue or error is occuring?
There isn't any documentation on this, however, we do not think there is a limitation on the number of lines allowed in a BPA.
We found in Bug 21672459, the customer was able to upload 900,000 lines to a BPA.
If this does not satisfy your question please let us know how many lines you are inquiring about.