WP All Import must be able to process this many records in less than your server's timeout settings. If your import fails before completion, to troubleshoot you should lower this number. If you are importing images, especially high resolution images, high numbers here are probably a bad idea, since downloading the images can take lots of time - for example, 20 posts with 5 images each = 100 images. At 500Kb per image that's 50Mb that needs to be downloaded. Can your server download that before timing out? If not, the import will fail.