*How to state a total for a list of items without any ambiguity?*

    I did a total of two thousand push-ups and sit-ups last month. At least.

Or

    I did loads of push-ups and sit-ups last month: two thousand at least.

*I have 6 legs, hands, and eyes.*

    I have hands, eyes and six legs.

You say, *Sometimes, you don't want to throw too many numbers at your reader.*

If you really want to tell your reader precisely how long you spent on each task, you may *have* to! The numbers may look better written as words though.

In context *I spent eight hours cleaning, cooking, and gardening* would usually make sense: if, for example, you are describing a single day. If you had indeed spent twenty-four hours doing those things  then you would surely say, *I spent twenty-four hours cleaning...*.

If the intended meaning is *I spent a total of eight hours...* then why not **use** 'total'?

    I spent a total of eight hours cleaning, cooking, and gardening.


*If I graded [approximately] 6,000 assignments, 4,000 midterms, and 2,000 final exams, how do I correctly state the total figure?*

    I graded [approximately] 12,000 papers: assignments, midterms, and final exams.

or 

    I graded assignments, midterms and final exams: 12,000 papers in all.

If they aren't papers, you could say *pieces of work* or *submissions* or whatever you call them.