I have not surveyed very much but as far as I know, the followings are the examples where the Zera meant the seed of human beings.
1. Genesis 38
8 And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother. 9 And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother. 10 And the thing which he did displeased the LORD: wherefore he slew him also.
2. Leviticus 15
16 And if any man's seed of copulation go out from him, then he shall wash all his flesh in water, and be unclean until the even. 17 And every garment, and every skin, whereon is the seed of copulation, shall be washed with water, and be unclean until the even. 18 The woman also with whom man shall lie with seed of copulation, they shall both bathe themselves in water, and be unclean until the even.
The other usage for zera is the seeds of plants, and mostly offspring, descendants, children.