11. conturbabimus illa, ne sciamus,
12. aut ne quis malus invidere possit
13. cum tantum sciat esse basiorum
11. We will jumble those things (what things?), so that we may not know,
Right, he is referring to the
basia, "kisses," with
illa. I.e. the kissing will be so chaotic that we
won't* know (how many there are)... "Won't" sounds more idiomatically correct in English, but if you are fond of subjunctives then go for it.
12. nor so that someone may cast an evil eye,
Right.
13. when he may know so many kisses to be..
"When" does work, but it might make more sense if you use "since."
So, all together:
We will mix those up, so that we may not know how many there are,
or so that anyone may not cast an evil eye
since he knows how many kisses there are.
Now, interpretation is a whole other game

I wonder if he means that there are so many kisses that no one would be jealous because it is far more kisses than anybody would desire, or if there is another meaning. Ideas?
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae