How to evenly portion chapati dough?

Even portions means same weight: a scale is the tool of choice here.

Bakers that want equal sized products just weigh the individual portions, then proceed from there.

The shaping / rolling of chapatis was already discussed in this Seasoned Advice post. Or you could use a press¹ to shape them.

If you are thinking of a larger scale of production, there are hydraulic chapati presses¹ or even (semi-)automatic machines¹ on the market, that shape chapatis of the same thickness out of equal sized (= weighed!) portions.

¹ No affiliation or recommendation, just random samples from the Internet!

When I'm trying to portion things evenly, I start dividing the sough into parts ... then smaller parts, etc.

So, for the dough I would :

1. make it into a ball
2. try to cut it into two or four equal parts, depending on the size.
3. roll each of those parts into a log / cylinder.
4. cut each of those in half, thirds, or smaller portions if you're comfortable. (narrower logs are easier to portion into more parts; it's best to cut in half, then half (or thirds) again, etc.)
5. repeat steps 3 & 4 until you have the desired number of portions.

I typically don't do this all at once. I might start with a quarter of the dough and portion, shape, and cook it, then move onto the next quarter, so it doesn't dry out too much.