How the dial peers are configured depends on the memory available on the router. Each dial peer requires approximately 6 KB of memory. However, if the MAX-DN is set to a high value, there is less space available to accommodate the dial peer. This is because the memory is reserved for that number of Directory Numbers (DNs). You can configure as many dial-peers as long as you dont use over 80% of the memory. So you must always have 20% memory free or it wont allow more dial-peers.
In order to resolve this issue, perform these steps:
Issue the show runcommand in order to verify the Max-DN value.
Reduce the Max-DN value to a value that creates room for more dial peers to be added to the configuration.