Do you have to see the results of your witnessing for it to count?
No. The one who witnesses does not have to see the fruit or be the person who sees someone saved. The responsibility is to sow the seed; another may water or add understanding, but God is the final cause and gives the increase. Faithfulness in sowing counts, not visible results.
Les explains that just as Paul planted and Apollos watered, but God gave the increase, so it is today. A believer does not have to always see the fruits of their witnessing and testimony, nor be the one who sees a person saved. One person sows the seed, someone else may come along and add to the person's understanding, and even a third person may bring them further. Regardless of how many are involved in an individual's salvation, God is the final cause, and God gives the increase.
I have planted, Apollos watered; but God gave the increase.
So then neither is he that planteth any thing, neither he that watereth; but God that giveth the increase.