Menu Tools-.Settigs

Here you can set up the number of Undo's and Printing.

Undo - Here you can specify the number of Undo steps.

Warning: The Undo buffers are in the memory, so if you have enouch memory and you are working on small documents you can eventually increase the number of undos.
However it is not recommended - it could lead to slowing down of the software or even crashes when all the memory will be gone.

If you want to know how much memory you have you just need to look at the right bottom corner of the PhotoSEAM window.

If the number is close to zero, you are getting into the trouble, save what you can, close documents and decrease the number of undo steps.

In case of very large documents you may even want to put 0, no undo. This could save a lot of memory.