In the upcoming version update, a series of items will be seeing an adjustment to their stack size, allowing for better inventory management. Elemental clusters which previously were unstackable will now be able to be stacked up to 12 clusters per stack, and several items ranging from Walk of Echoes coins to fishing bait will see their stack sizes increased from 12 to 99 per stack.

For a complete list of all items seeing stack adjustments, check below the link, and disuss this on the Zam Forums!

From the official Final Fantasy XI forums:

The upcoming version update will see the stack size adjusted for certain items.

Changed from 1 to 12

Fire Cluster
Ice Cluster
Wind Cluster
Earth Cluster
Lightning Cluster
Water Cluster
Light Cluster
Dark Cluster

Changed from 12 to 99

Coin of Birth
Coin of Advancement
Coin of Glory
Coin of Decay
Coin of Ruin
Slice of Bluetail
Peeled Crayfish
Slice of Carp
Sardine Ball
Crayfish Ball
Insect Ball
Trout Ball
Meatball
Lufaise Fly
Sliced Sardine
Sliced Cod
Peeled Lobster
Lugworm
Little Worm
Drill Calamary
Dwarf Pugil
Pebble