You'll need a PDF API for C#. iTextSharp is one possible API, though better ones might exist.
iTextSharp Example
You must install iTextSharp.dll as a reference. Download iTextsharp from SourceForge.net This is a complete working program using a console application.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using iTextSharp.text.pdf;
using iTextSharp.text.xml;
namespace GetPages_PDF
{
class Program
{
static void Main(string[] args)
{
// Right side of equation is location of YOUR pdf file
string ppath = "C:\\aworking\\Hawkins.pdf";
PdfReader pdfReader = new PdfReader(ppath);
int numberOfPages = pdfReader.NumberOfPages;
Console.WriteLine(numberOfPages);
Console.ReadLine();
}
}
}