I have created a chatbot using Dialogflow fulfilment (actions-on-google node.js library) and added 8 suggestion chips, but all the chips are shown horizontally in a single row.
Is there any way to show 4 chips in one line and another 4 chips in the second line. so that user doesn't have to scroll.
Eight suggestions are... a lot. Typically you wouldn't want more than about 5, and even that might be a lot in some cases.
To answer your direct question, however - you don't have control over the layout of suggestion chips. How many are visible depends on the screen size and orientation, and future versions of the Assistant may choose different ways to represent them.
While you may wish to use a List visual layout, this is mostly good for more dynamic responses (returning a list of titles) rather than a menu. It also requires a different kind of handling for the reply.