How to map multiple existing term IDs to an entity
You need to use plugin sub_process. File yml have soure like this:

  field_prod_category:
    plugin: sub_process
    source: tids
    process:
      target_id: tid


and soure plugin need to prepare:

$tids = [
      'tid' =&gt; 806,
      'tid' =&gt; 635
    ];
$row-&gt;setSourceProperty('tids', $tids);