Is NEWSEQUENTIALID unique within a database for Many tables? Is NEWSEQUENTIALID unique within a database? We plan many tables in a database with NEWSEQUENTIALID, inserting simultaneously and want to ensure every guid will be unique, regardless if they are in different tables. 

Regular NEWID() will guarantee uniqueness in a database, and chances of having duplicate is chance of being struck by asteroid/lightning, however they do not alleviate the fragmentation issue.

This article talks about Mac address, however I want to specify question for database specific above.
https://stackoverflow.com/questions/28404964/is-uniqueidentifier-unique-across-databases
Acording to the documentation from Microsoft.

NEWSEQUENTIALID


  Creates a GUID that is greater than any GUID previously generated by
  this function on a specified computer since Windows was started. After
  restarting Windows, the GUID can start again from a lower range, but
  is still globally unique


I recommend using NEWSEQUENTIALID over NEWID() for any case that that you aren't relying on the built in randomness, mainly due to the fragmentation issues associated with random GUIDs. There is a fantastic article by Kimberly Tripp on some of the pitfalls of using NEWID() on a Primary key.

There are a few situations where NEWSEQUENTIALID may not guarantee  uniqueness as are listed in the documentation.


  The UuidCreateSequential function has hardware dependencies. On SQL
  Server, clusters of sequential values can develop when databases (such
  as contained databases) are moved to other computers. When using
  Always On and on SQL Database, clusters of sequential values can
  develop if the database fails over to a different computer.


However judging by the phrasing of your question it seems like this will be contained on a single database server.

It is also worth noting(not that it applies here) that NEWSEQUENTIALID can be predicted so if privacy of that GUID is a concern this could also be a problem.