I want to upload large number of ECW files to GeoServer. Do I have to do it one by one, is there a way to upload all ECW files at once?
You can use the GeoServer REST API to mass configure layers (one call configures one, script a loop to cover them all).
That said... Last time I checked using ECW on the server side requires a commercial license (for the usage of the ECW decoder server side, GeoServer itself is free). I assume you have one. If not, converting them to GeoTIFF, particularly in COG format with the right compression, might get you out of trouble.