Resolution will be based on Nintendo Investor Relations data.

https://www.nintendo.co.jp/ir/en/finance/hard_soft/index.html

As of the March 31st, 2023 update to their numbers, the DS sold 154.02 million units, and the Switch has sold 125.62 million units so far.

If Nintendo report that the Switch has outsold the DS at any point before 2026, this market will resolve YES. If they don't, NO.

The final chance will be the FY2026 Q3 sales update (likely to be released early February 2026), which should have data for unit sales up to December 31, 2025. If the Switch has not overtaken the DS according to that report's numbers, this market will resolve NO at that point.