A meld can not maintain greater than seven playing cards. So to “end” a six-card Jack meld, you solely want another Jack.
From the guidelines on Pagat:
A meld is a set of from three to seven playing cards of equal rank positioned face up on the desk. A meld can not have fewer than three playing cards or greater than seven.
A meld can not include greater than seven playing cards, and a partnership is just not allowed to have two incomplete melds of the identical rank, however if you happen to full a pile, you may then begin one other meld of the identical rank.
If you are making an attempt to select up from the discard pile to finish the meld, the foundations go on to deal with this case explicitly:
Therefore when you have an incomplete meld of 5 or 6 playing cards on the desk, you won’t be able to select up a card of that rank from the discard pile except you could have sufficient playing cards of that rank to complete the primary seven card pile and make a brand new three card meld of the identical rank. These playing cards should all come out of your holding and the highest card of the pile – once more you aren’t allowed to utilize different playing cards you’re about to select up from the discard pile to fulfill the requirement.