How many luggage (suitcase) are we allowed to bring with us?

Generally, one traveler could have one carry-on Baggage with weight less than 20 kg (44 pounds).

Thank you.
How much will it cost to bring one additional luggague per traveller?

In fact I don't think they have limitation for the pieces. They said 20kg in total, however, this is not very stric as well. You can take the luggage with you. If they question about the weight on the train, you may need to pay an extra fee. Not very sure about the detailed charge. If you want to arrange the checked luggage, it should be HKD4 per 5kg and also around HKD50 per piece handling charge.