Learn Arabic (Modern Standard): الطرد رقم ستة ثقيل.

When you use a number as a label or an ID, like "number six," you use the bare counting form of the number. For numbers 1 through 10, this default counting form always ends with the feminine ة (taa marboota). You don't match the gender of the noun here because you aren't counting a quantity of "six parcels."

How do we know this means "is heavy" and not "the heavy parcel"?

The word for heavy, ثقيل (thaqeel), does not have the "the" prefix ال (al-), while الطرد (the parcel) does. This mismatch tells us that "heavy" is giving new information about the parcel, acting as the second half of an "is" sentence. If you wanted to say "the heavy parcel," both words would need ال.

Can I just say الطرد ستة without the word رقم?

It is much more natural to include رقم (raqm), which means "number." Just like in English, saying "the parcel six is heavy" sounds a bit awkward. Adding رقم makes it perfectly clear that six is the ID or label of the parcel.

How is the taa marboota at the end of ستة pronounced in this sentence?

Because it is a bare number acting as a label rather than the start of a possession phrase, the taa marboota at the end of ستة is pronounced as a soft "ah" sound (sitta), not "at".