Jennis' vs Jennis's

Assuming the name is Jannis, then it would be "Jannis's toys".
"Jannis' toys" implies the toys belong to a plural of Janni.

Both are correct though Jennis' toys is more used.

khoshtip: Jennis' toys is more used.

Have you any evidence for that statement?

I think both can be used. Please correct me if I am wrong.

tamguatlayI think both can be used. Please correct me if I am wrong.

Some people use only the apostrophe in classical and biblical names, Jesus' disciples, Socrates' death, but most British writers use 's all the time: James's parents, Jesus's disciples.

You would normally write it the way you would say it. If there is an extra syllable at the end in normal speech, use "s's" at the end. For this reason, I would use "Jennis's toys".

fivejedjonHave you any evidence for that statement?

I talked about the use. In all of those many English books I read and websites I deal with (which are all in English), I haven't seen that kind of using possessive s, even one time.

khoshtipI talked about the use. In all of those many English books I read and websites I deal with (which are all in English), I haven't seen that kind of using possessive s, even one time.

See:

Chalker, Sylvia and Weiner, Edmund (1994.30), The Oxford Dictionary of English Grammar, BCA/OUP
Greenbaum, Sidney (1996.550), The Oxford English

You listed your grammar books, regardless of attention to what I'd said. About the grammar and rule on that topic, there isn't any right answer to that question and that was why I emphasized the subject of the use. What I said was what I have experienced from my books. These are some of them:

Computer Networking A top-down Approach 6 edition by Kurose and Ross.

Thanks, khoshtip.

The books you listed don't seem to be books on English usage.