Avast won’t scan inside a folder with a big number (hundreds) of files.
Avast scans folders with tens of thousands of files inside without any problem. Please elaborate a bit about the exact nature of your issue.
When I open a folder with a big number of files, like ‘System32’, mark all files and right-click ‘Scan selected files’, Avast won’t start the scan.
I rather think there may be more than many hundreds, on my win10 C\Windows\System32 folder which is 8.63GB it scanned through that whilst I was formulating this reply.
You don’t need to mark all files just use the right click menu (on the C\Windows\System32 sub-folder) and select ‘Scan selected items for viruses’.
I see… I’ll check if anything has changed, it’s possible that there’s some limit when passing the selected files from the Explorer extension to the Avast process.
In any case, if you want to scan all the files inside, just select the parent folder (e.g. System32) instead and let that one be scanned (as David suggested).