"That" is referring to "what I'm referring to", not the objects themselves. Das, was ich meine (what I mean/I'm referring to), ist ein Apfel/sind zwei Äpfel. That, what I'm referring to, is not one apple. That, what I'm referring to, are two apples. Das ist eine Frau. Das ist ein Mann. Das ist ein Kind. Das sind Menschen.

The problem is that many learners compare the language they study with their mother tongue. In English people say either It is an apple or This is an apple = Das ist ein Apfel. In plural These are apples or They are apples = Das sind Äpfel. So only "das" is used both for singular and plural in German.

No, it's fine to use "das" as the 'dummy subject' in such sentences even with plurals. Incidentally this was also the subject in a "Fragen Sie Dr. Bopp" article just a few days ago: http://canoo.net/blog/2016/12/07/wenn-das-verb-nicht-mit-dem-subjekt-uebereinstimmt-das-sind/