Bulk Rename Utility

I select approx. 18000 files in Windows Explorer to rename with BRU,
I get an error displaying "too many files selected".

What is the maximum of files to select?

When using the Windows File Explorer extension Bulk Rename Here, the limit is the total sum of characters contained in the path multiplied by the number of files, so there is no specific limit for the number of files, it depends on path length too.

OMG. Could the Dev's fix that?

the limit is the total sum of characters contained in the path multiplied by the number of files

I believe that's going to be 4096.
(There are ways around that.
BRU might need to create a .dll that is "registered" & used as a context menu "shim" that bypasses Windows limits, & is then able to pass large number of files/path lengths to BRU. [IrfanView offers an IrfanView Shell Extension (as one method) for that. Another, it allows reading of filenames from a text list [/filelist=].)

If all the files are in a single directory, or single directory tree, you could open to that directory in BRU & that would work.

If all the files are in a single directory, or single directory tree such that you can select them all, you can drag & drop them into BRU & that would work.

If the files are "haphazard" (i.e., all over the place) & if you can search & find the wanted files in some manner (perhaps with Everything search), you can find them using Everything & drag & drop them into BRU.