Use Attribute Assistant to do the auto numbering. It comes as part of the Local Government Information Model (LGIM) templates like the Address Data Management template (you don't need to the the LGIM). To create an auto increment number use the Attribute Assistant toolbar, the GenerateID table and DynamicValue table. Press the download button to get to the link to download it.
After extracting the files double click the esriAddin files for Attribute Assistant and Address Management to install them while ArcMap is closed. In the GenerateID table you add a row with the feature class name, the current number of the sequence in your table. Use 0 if you are starting with nothing numbered, but this only works for blank feature classes/tables. Use the last existing number of an existing sequenced field if it already has records (see the autoincrement calculation to initialize an existing long field).
|OBJECTID *||Sequence Name||Sequence Counter||Interval Value||Comments|
Then in the DynamicValue table you enter the feature class/table name holding the feature class name (WHYDRANT), the field to be incremented (FACILITYID), the Value Method (GENERATE_ID), a Value string (WHYDRANT|0|[seq] - meaning increment a normal number with no leading zeros stored in the WHYDRANT feature class), and make On Create True and all other options false.
|OBJECTID *||Table Name||Field Name||Value Method||Value Info||On Create||On Change (Attribute)||On Change (Geometry)||Manual Only||Rule Weight||Comments|
Add both of these tables to your map from the directory you extracted it to in the \AddressDataManagementLG\MapsandGeodatabase\LocalGovernment.gdb. Also add the feature class (like wHydrant under the WaterDistribution dataset). Now add the Attribute Assistant toolbar (right click toolbar area and choose Attribute Assistant) and make sure that the first button is active (the red exclamation mark in the icon disappears). Add a new feature and notice the next number in the series for your field was added.