As far as I recall:
In former times there was a product named PDM (Persistent Data Masking) which essentially consisted of the TDM software (Test Data Management). The "contrary" to PDM was DDM (Dynamic Data Masking); this tool was used to mask data visible in certain online applications (for example, a web browser UI for insurance companies) without the need to touch the actual database contents.
In other words, DDM was used to mask data presented on a screen; PDM was meant to mask data in physical storage (mostly databases, files, and R/3 systems at that time).
The product name PDM has been abandoned some time ago, as far as I know, and I don't even know whether DDM is still sold. Simply use the term TDM for persistent data masking, that's absolutely fine.
The question how many DB users you need is not easy to answer for several reasons.
First there are some basic recommendations from Informatica about user IDs for any kind of repository DBMS because each DBMS has its own "standards" for user names, database / schema setup, and the like.
For example, it is not recommended to use one single Oracle user ID for all repositories in a TDM environment, but technically it should work fine as long as your DBA provides enough separate user schemas.
In SQL Server, however, it is recommended practice to set up one single database for each repository in the system, but you can easily access all these databases with the same user ID (whether you want to use different schemas per database or different databases is up to you, but my personal recommendation is to really use separate databases for each repository instead of using separate schemas within the same database).
And so on. Technically you can work on most DBMS with one single user ID; whether your organisation considers this safe and sound, that's a completely different question.
Second it depends on which parts of the TDM suite you want to use. If you're installing the complete stack with PowerCenter, two model repositories, and so on, then you can easily use 6-7 different user IDs and/or separate database schemas / databases. If you just want to mask data and don't intend to use the Analyst tool and so on to first profile your data, then you only need three databases / schemas / user IDs, namely for the domain repository, the PowerCenter repository, and the TDM repository.
It depends on what your organisation plans to do.
As you can see, it's easy to discuss these things in lengthy detail. I won't continue that here. May I just suggest that you look for someone experienced in this matter who can help you with the analysis in your organisation.