If you're returning more than 20 hitsPerPage for a single AutoComplete source and sending events with the AutoComplete Insights Plugin you may run into a 422 Error: Cannot process more than 20 objects in an event.
Previous AutoComplete versions did not have built-in batching with the Autocomplete Insights plugin. However as of version v1.8.1 of AutoComplete this has changed. Updating to this version will resolve the error.