public static int GetNumberOfPages(string pdfData)
{
    var regx = new Regex(@"/Type\s*/Page[^s]");
    var matches = regx.Matches(pdfData);
    return matches.Count;
}

Open a PDF document and store the data in a string. Provide that string to this function and it will return the number of pages in the PDF.

Get number of pages in a PDF file

C#