The outlined numbers break up because the font's digits are built from overlapping shapes. Each option below avoids outlines. Two chapters are shown so you can see how they read in sequence.
The same large number, filled solid in pale mist-teal instead of outlined. Keeps the editorial feel, quiet behind the heading.
A teal circle with the number, joined by a thin line running down the page from chapter to chapter, so the story reads as a journey.
The large number filled with teal halftone dots, the motif from the NPoC logo. The most distinctive and on-brand.
No big number: a small numbered circle and "Chapter" label above each heading. The calmest, letting the photos and headings lead.