counter for waves (of a repeated occurrence)
「波」 has other readings with different meanings.
感染症は第三波を迎えたと報告された。
The infection was reported to have entered its third wave.
大きな波が来ているから注意して。
A big wave is coming.
今回の寒波は二波に分けて来るらしい。
This cold snap will apparently arrive in two waves.