Keyboard shortcuts can be configured by editing your ~/.config/xfce4/terminal/accels.scm
file.
If you want to disable Alt+<N> shortcuts that switch between tabs, do not comment out or delete the corresponding entries in the “accels.scm” file. Just replace the field values with blanks: “”.
When experiencing bugs in Xfce Terminal, your way of helping things getting fixed is to report a bug about it in bugzilla. Note: For this you will need to have/create an account.
Before reporting a new bug, please always check the list of open bugreports to avoid duplicates.