Although,I do not see that CR should have any limitation on the number of formula fields;I do personally feel that it is a bad practice and,on invoking report from the web,performance issues would become significant as it might lead to significant delay in displaying the report.
If possible,use minimum number of formula fields and , allow major tasks to be performed directly on the Server,rather than the client PC.
Incase,you are using Database like SQL Server or Oracle,you can make use of VIEWS ;instead of tables to reduce the load.
Pooja M.,
http://home.attbi.com/~pooja.mohan