Serge,
The formula

=COUNTBLANK(A1:G1)<=6

Will return TRUE if you fill more than one cell in A1:G1. This formula
can be used in Data Validation (Custom), in Conditional Formatting
(with Formula Is:) or in a single cell next to the data as in:

=IF(COUNTBLANK(A1:G1)<=6, "", "Not more than one entry")

HTH
Kostis Vezerides