How many datasets can I create?

There are no limits to the number of datasets you can create.  Each dataset can store large sets of records and fields. Datasets can be very helpful if you need to organize very large sets of data or you need to limit access to certain records, but an creating an additional dataset isn't always the best solution. 

For example, a high school may have 9th, 10th, 11th, and 12th grade students, however the school does not need to create 4 datasets to store their information since all students could be stored in one dataset along with their grade level.  Having additional datasets would likely increase the complexity without much benefit. 

If, however, a school district had multiple schools they may want to create a dataset for each building.  This may make it easier to manage the data and allow the district to restrict access for a dataset to only the staff that work at that school.