If you have seen both numbers, neither source is wrong; they count different things. Under the current EASA syllabus (the ECQB question bank generation, often called the new ATPL syllabus) the exam set is 13 examination papers: VFR and IFR Communications, which used to be two separate papers, are now examined together as a single Communications paper. The older syllabus had 14 papers, which is why many schools, forums, and question banks still say 14 ATPL subjects.
SkyStudy organises practice into 14 practice subjects because VFR Communications (091) and IFR Communications (092) stay separate in the app: the phraseology and procedures differ enough that drilling them separately works better, even though you sit them as one paper.
The Knowledge, Skills and Attitudes component (KSA, area 100) does not change the count either: it is assessed through your ATO and scenario-based questions woven into the other papers, not as a separate examination.
The examination rules behind this (attempt, sitting, and window limits under FCL.025) are published in the EASA Easy Access Rules for Aircrew and on EUR-Lex (Regulation (EU) No 1178/2011). To map the 13 papers onto sittings within the FCL.025 limits, use the free ATPL exam planner, and when you are ready to practise, the ATPL question bank covers every subject with explanations. For how the knowledge itself is organised and studied (notes, question practice, and what self-study really means under EASA rules), see the ATPL theory guide.