What is the relationship between a changeset and its tags? to give an example; can a ChangeSet have multiple Comments? or Multiple other tag Key/Vals? thanks

As with nodes, ways and relations, a changeset can have many tags. Each key is unique, so there can only be one tag with the key "comment", for example.

As for approaches for storing this in a SQL database, the two main ones are to have a ChangesetTag table, with foreign keys to a Changeset table, or alternatively use a dynamic storage column (e.g. json type) called tags on the Changeset table. Both approaches are used in different parts of the ecosystem.

You can only have one tag with "comment" as a key, but you can have multiple tags on a changeset, with different keys, eg :

<changeset id="123" key1=val1 key2=val2 ... etc>
  <tag k="comment" v="Comment-1"/>
  <tag k="comment2" v="Comment-2"/>
  <tag k="source" v="some sources..."/>
</changeset>

If you care only about the comment tag, you can store its value in your ChangeSet table. Otherwise you need to have another table with key, value and reference to the changeset (or json store as Andy said).

What you saw (I guess), on the ChangesetMD page are other comments, displayed on the changeset page under the header "Discussion". These are not tags, and are stored differently.

The xml, from the documentation, looks like this :

<osm>
    <changeset id="10" created_at="2008-11-08T19:07:39+01:00" open="true" user="fred" uid="123" min_lon="7.0191821" min_lat="49.2785426" max_lon="7.0197485" max_lat="49.2793101" comments_count="3" changes_count="10">
        <tag k="created_by" v="JOSM 1.61"/>
        <tag k="comment" v="Just adding some streetnames"/>
        ...
        <discussion>
            <comment date="2015-01-01T18:56:48Z" uid="1841" user="metaodi">
                <text>Did you verify those street names?</text>
            </comment>
            <comment date="2015-01-01T18:58:03Z" uid="123" user="fred">
                <text>sure!</text>
            </comment>
            ...
        </discussion>
    </changeset>
</osm>