I'm using Sitecore 9.1 and SXA 1.8, there are two cores for the master index now ,
instance_name_master_index, which index should I put the SXA autosuggester configurations at Show Predictions mode.
Well, it should be configured on all indexed where you want to use predictions. If you are still developing your website or styling it you are probably using the master database in editor preview mode. If you would like to have the preductions show there update core for
If your website is ready (no more development or need to have predictions working in the editor or preview mode) configure it only on the core used for
sxa_web_index (or any other core which you are using on your CD server).