How can an administrator see what  tokens of a particular type are assigned or unassigned.
custom query software token.

An unassigned token has a value of SDToken.iUserNum = 0, an assigned token has the non-zero user number.

Various types of tokens have their own values for  SDToken.iType, an incomplete list of token types follows:

0: Standard card
1: PINPad
2: key fob
4: Software token
9: SID800
99: user password 

Since the software token has an iType of 4, a basic SQL query can be:   

SELECT  chSerialNum from SDToken WHERE iType = 4 AND iUserNum = 0

This sample query will display the serial numbers of unnassigned software tokens. You are not limited to displaying chSerialNum, you can easily customize this sample query by adding any additional required fields from the SDToken table. Please note that this sample query is simply checking for unassigned tokens; it is possible that a previously assigned software token was also issued, but when the token was Unassigned, it was not also Revoked. 

Customizing: Changing the iType will change the type of token referenced. If you want to see assigned tokens, change iUserNum > 0