There are 39 categories of forbidden melochos on Shabbos. The number 39 is derived from אלה=36 + דברים= 2 + הדברים=1.

Although logically one would think the 39 melochos are different forms of the same sin (and therefore one who violates multiple melochos should bring just one Korban) we derive that actually each melocho is distinct and requires it’s own Korban.

This concept of חילוק מלאכות has few sources. Either bc the Torah singles out melocho of kindling a flame to show melochos are distinct, or bc it says מות ימות and we apply the double language to say ones money is “hit” twice as they must bring two Korbanos for two melochos, or from the language of אחת מהנה that implies one sin can obligate multiple Korbanos.

Even though there a concept of חילוק מלאכות, that is all when someone forgets about the melochos. But when someone is aware of the melochos and does multiple melochos bc he forgets it Shabbos, there’s only one Korban. The Gemara explains the rationale is that the Torah obligates a Korban bc of the forgetting of the law, and in this case, there was only one thing fogotten.

What is someone is unaware that it is shabbis and they only don’t know multiple melochos are assur? When they perform a few melochos, do we treat like שגגת שבת and one Korban suffices or like שגגת מלאכות and they need multiple korbanos? The gemara is back and forth on this but ultimately we favor the side that one korban suffices.