May I know the upload files limit inside a folder?
I mean each folder can contain how many files ?
There is no such fix number for files to upload inside a folder.
For SharePoint Online:
According to the Microsoft's official documentation: Service limits for all plans.
Items in lists and libraries - A list can have up to 30 million items and a library can have up to 30 million files and folders. After 100,000 items are added to a list, library, or folder, permission inheritance for the list, library, or folder can't be changed. To learn more about other restrictions for viewing large lists, see Manage large lists and libraries in Office 365.
File size and file path length - 15 GB. The maximum size for files attached to list items is 250 MB.
Also, According to the article, Ultimate guide to SharePoint size and usage limitations:
Document libraries should not contain more than 30,000,000 documents (supported). That’s an insanely large amount. You can go larger than this amount, but Microsoft doesn’t recommend it.
By default the file size limit is set at:
SharePoint 2013: 250 MB (supported), though your IT department can increase that to a hard maximum of 2 GB (boundary). But that 2-GB value is an absolute maximum. Unlike the 30,000,000 suggested limit above, the 2 GB file size can be very limiting at times, especially if we’re talking multimedia and video.
SharePoint 2016: 2 GB (supported), though your IT department can increase that to a hard maximum of 10 GB (boundary). But that 10-GB value is an absolute maximum.
SharePoint Online: 15 GB (boundary), only if you drag-and-drop the file. Other upload methods (e.g., the "upload" button) are limited to 2 GB.
30 Million is upper limit, whatever folder structure you have and number of folder you have....all together should not exceed 30 Million.