I assume you know because they just won't sleep for one nap?
No, it comes when baby starts sleeping longer. So then there just are fewer naps in the day needed, rather than it being a deliberate nap refusal.
For example a baby sleeping 30m and having 90m awake time will have a day like this:
8.30-9am nap
10.30-11am nap
12.30-1pm nap
.... and so on.
Then one day baby goes to sleep as normal for a nap at 8.30am, but doesn't wake until 10am. In which case a 2h awake time takes you to 12pm for the next nap - and there you go, already one less nap than in the example above.
You usually get 3-nap days established first (morning, lunchtime and teatime). Mine was 5 months when this happened. I had spent time deliberately forcing extended naps from very a very early age. I started getting nap lengths of 2h at that point.
Dropping from 3-naps to 2-naps is more tricky, that often needs an extension of awake time that baby can manage. This happened about 7-8 months with my youngest and we stayed at 2 naps until 20-22 months old.