兩個做種 Two Kept Back For Seed
即往腰間順帶裡摸摸，還有十二個。「送他十個，還留兩個做種。」即將蟲兒拋了去，散在十個小妖臉上，鑽入鼻孔，漸漸打盹，都睡倒了。即往腰间顺带里摸摸，还有十二个。「送他十个，还留两个做种。」即将虫儿抛了去，散在十个小妖脸上，钻入鼻孔，渐渐打盹，都睡倒了。
逐句解 Line by line
He felt about in the pouch at his waist: there were still twelve of them.
順帶 is the belt-pouch, and the verb is doubled — 摸摸, a bit of a rummage. The Great Sage patting his pockets.
"I shall send him ten, and keep two back for seed."
做種, to keep for seed, is a farmer's phrase: the portion of the crop you do not eat so that there will be a crop next year. He is treating magical insects as agricultural stock.
He threw the insects out, and they scattered onto the faces of the ten little demons and bored into their nostrils, so that little by little they began to nod, and all of them dropped asleep.
倒 is dǎo here, the falling-over reading, not dào. The pacing is careful: 漸漸打盹, gradually nodding, then all down at once.
Ten insects for ten guards, and two held back. The arithmetic is the passage, and it is arithmetic Wukong does out loud.
Keeping two for seed is a farmer's instinct and it tells you something about this character that the fights never do. He is not hoarding treasure; he is thinking about the next harvest. Whatever else the Great Sage is, he grew up on a mountain and knows you do not eat everything you have.
It is also, quietly, a piece of plot engineering. Two spare insects have been placed on the table and named, and no reader who has got this far expects them to stay in the pouch.