The data can be incorrect if, for example, you have 6 layouts and delete one. The 6th one’s data remains in the database. There isn’t any way to automatically delete ACF data in the database that is no longer used. You’d need to create your own acf/save_post filter and do checking based on your field groups and fields to remove anything that shouldn’t be there.
You can get an accurate count of the layouts on a site. The meta value 'flexiblefield' holds a serialzed array. If you get all of the meta values under this meta key the total count of all of the arrays will be the total count of all layouts for that flex field.
