Only one undo is allowed in layout mode and all other modes. Can't say why, you'd have to ask the FileMaker programmers that question. It is possible to request new features if you care to do so: http://www.filemaker.com/company/feature_request.html
I have submitted a feature request because this is just nuts.
I don't care about trying to undo script deletions or major objects, but I do need more than one level of Undo for typing and formatting changes.
An example of a common problem I have is accidentally using the format brush on the wrong object. While I am a skilled touch typist, I may use four or five different keyboards a day, so when I try to hit CMD-Z, I may hit the wrong combination. That uses up my one level of Undo and I cannot make the correction I originally intended.
Most sophisticated programs have user settable levels of Undo to balance memory requirements against ease of correction. I cannot imagine this would be too difficult to implement since the programmable UNDO function already permits multiple Undo in scripts.