The assignments are tough because each of them is really heavy while the number of them are fine.
The number of them are ok but each of them takes me a lot of time.
It's so hard to complete the assignments because each of them is to hard to finish regardlerss of the number of them.
～が大変は
It is tough that~
It is tough to ~
It is hard that ~
It is hard to ~
課題の数
the number of assignments
ひとつひとつの課題
each of them(assignments)
重いは、そのままheavyでも通じますし、2番のように
take time : ~時間がかかる
の形も使えるでしょう！
It's not the number of assignments that's hard, but the amount of work needed to complete them.
There aren't many assignments but they take a lot of time to complete.
It's not the number of assignments that's hard, but the amount of work needed to complete them.
「大変なのは課題の数ではなくて、課題を完成するの努力のことです。」
Amount of work(/effort) needed - 必要な努力.
There aren't many assignments but they take a lot of time to complete.
「課題の数は少ないけど、多くの時間がかかります。」
Take a lot of time - 多くの時間がかかる.
日本 
オーストラリア 
