There isn't a rule of thumb that says that it is bad to have lots of tables, it's a matter of how your data is structured.
If you have a layout based on Table A and it has a lot of data from Table D on it, and that data is retreived by going from Table A -> Table B -> Table C -> Table D then that is going to slow your system down.
Having multiple tables can actually help speed your solution up. This is something that Mark Richman has spearheaded: http://www.skeletonkey.com/blog/mark_richman_filemaker_developing_for_maximum_wan_performance
The short of it is that if you can move large chunks of data that aren't frequently accessed into additional tables and only access that data when it's needed then you can speed things up significantly.