A memory map of all treasure chest data.
http://www.chronocompendium.com/wiki/index.php?title=Text_List_of_Treasure_ChestsThere are 249 in the game.
Subtract 5 that are only used as an index to load data from another room
Subtract 4 that can't be obtained without cheats (walk through walls)
Total: 240 that that can be opened.
This list and the number above is of only "real" treasure chests that use the data at $35F000
It is possible to create a fully functioning treasure chest using only event code (another example at the amount of freedom the event code offers). Several locations are programmed this way, ex: 044 Northern Ruins, Object 08. Sealed chests are also created using event code. Both types use storyline bits in another memory range.
Finally, it appears memory range 7F0020-7F004F (48 bytes) is extra space reserved for additional treasure chests. It would be possible to use this as memory expansion space similar to the empty item inventory slots, provided you don't plan on creating more treasure chests that could potentially conflict. You could, for example, use 7F0030-7F004F for your own purposes, leaving 7F0020-7F002F for treasure chest expansion (127 slots, which is obviously a lot)
Have fun.