I want to give a projection for multiple PNG files at the same time i.e. not one by one.
Is there a tool for this or can we give a projection to that folder?
As commented by @MichaelStimson the way to do this is by using the batch grid:
Right click on the Define Projection tool and select 'batch' then drag and drop your PNG files from ArcCatalog or Explorer into the tool, define the CRS you want to give in the first row then right click and select 'fill' and all others will be populated.
For more information see the Online Help page titled A quick tour of batch processing.