Hi C3Projects,
There is no limit on the number of pages can be added to PDF.
Could you please let us know if you are trying to create a PDF from other format files or you trying to combine the pages of different PFs to one PDF?
Please share the screenshot of the error message you get when you add the pages to PDF.
To add the screenshot here, please use the insert icon provided on the reply window as shown below:

Once you click on this icon, you get a small window.
Click on Insert Image and then browse the location where you have saved the screenshot file.
Select the file and click on Open > Insert Image.
We will be waiting for your response.
Regards,
Meenakshi