That depends on what version of ACF your using and where you are adding the value.
If you’re adding the value to a post then it will be in the _postmeta table. If you’re adding to a options page then it will be in the options table. If you’re adding it to a term, in ACF < 5.5.0 it will be in the options table, in ACF >= 5.5.0 it will be in the _termmeta table.
