There is no built-in limit on the number of records.
However, the current implementation makes extensive use of computer memory resource. That puts a practical limit depending on the amount of memory (RAM) in one's system. Additionally, the properties of analyzed data affect how much memory is used. For example, 1000 review articles with long lists of cited references would require significantly more memory compared to 1000 records with only a few cited references. Further, the number of local (within the analyzed set) citations may require additional memory usage. Therefore, there is no set answer to this question.
As a point of reference, we have analyzed collections of about 20K records.
Future versions of HistCite should use less computer memory.