If you must use sequential numbers, you should have the database assign a temporary number to the document when it is created and then have only one server execute an agent that assigns permanent sequential numbers to the documents on a daily or more frequent basis.
However, most of the time people just need UNIQUE numbers assigned to the documents. Using the @Unique formula generates a unique string to identify a document. Or, you could have it assign sequential numbers that include the server name as a prefix. You could use a combination of date-time and server or user information to create a unique identifier as well.
My experience is that most of the time when people say they have a requirement for sequential numbers, they're wrong, they just need unique numbers and think that sequential is the only way to do it.