Hi @Anonymous ,
as always, it depends 🙂 without really looking into portals, you have at least websites, webpages, content snippets, (site settings), page templates, web tempaltes, notes, attachments, webroles, entity permissions, entity forms, entity form metadata, webforms, entitylitsts, access control rules, website languages, (portal languages), web links, web link sets, site markers, the m-n relations between entity permissions and webroles (other m-n relations??)
these already make up more than half of the 30. If i look in the dynamics navigations, there are some more things (well, contacts should not be transported 🙂 ) so this number sounds reasonable.
I never used the configuration/migration tool but when it respects the ids (guids) of the entries there should be no problem of overwriting stuff.
Nevertheless:
When we transport portalstuff there are things, we only transport initially. Especially site settings are in my opinion "site settings" which usually differ between systems. Some snippets may also be not great to transport, for example navbar left may be "test system" in one case and "mycompany" in the other system (but i think you have no choice to transport only specific entries)
Hope it helps and points in the right direction,
Christian