The first problem with the query is the term "libraries." This is usually taken to mean "partitioned data sets." There are many other types of data sets that can be mixed with "libraries."
You use DD statements to specify a "library" data set or any other sort of data set to a job step.
The proper way to make this query is to ask, "How many DD statements can I specify for a job step?"
Unfortunately, there is no hard answer to this query, as it depends on the maximum size of a data area in the operating system called a TIOT, and this is set by system administrators when the system is set up. Another complexity is the space required for a DD statement in the TIOT is not fixed.
However, in most systems, you can define approximately 1500 DD statements for one job step. In a few systems it may be lower, and in some systems it may be as high as approximately 3000.
There is a hard limit on the maximum number of job steps in one job: 255.