Hi Stephan,
I can suggest you 2options,
Options:1
You have a option in crystal report, for example A/R invoice have 11 lines. You can print first 10lines in page one other 1 line in page 2 and 9 empty lines for cover the full page.
If the customer satisfy this print then you can achieve your requirement through crystal reports.
Option:2
Write Stored Procedure where you needs to restrict. By default its not possible through any settings. Below is sample SP for A/R Invoice
if @object_type = '13' and @transaction_type in ('A','U')
begin
if exists(Select distinct 'Error'
from INV1 W1 where w1.LineNum > '9' and W1.DocEntry = @list_of_cols_val_tab_del)
begin
SET @error = 1234
SET @error_message = 'Invoice should contain below or equal to 10 lines'
end
end