Must one make up for the Maghrib prayer that was shortened out of ignorance of its ruling while traveling, and how is it to be made up, especially given the inability to determine the number of days it was shortened?

There is a consensus that Maghrib prayer is not to be shortened. Whoever shortens it out of ignorance must make up for it in all cases where he shortened it, provided he can identify those instances. If he does not know their number, he should continue making up for them until he is reasonably certain that his obligation has been fulfilled.