Here is the reason why Smriti Mandhana wears jersey number 18

She wanted number seven as it was her roll number in school

Last Modified Thursday, 7 February 2019 (16:00 IST)
is significant for two reasons, first India's captian wears it and second this is the number of Smriti Mandhana's jersey. She told Yuzvendra Chahal that had no idea that Virat Kohli too wears it.
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Why jersey number 18, Mandhana clears out saying she wanted number seven as it was her roll number in school but that was taken by someone else. A BCCI manager suggested her to opt for number 18 as this is also her birthdate in July month. When she chose to wear jersey number 18, she was fully unware that Kohli too wears that.  
 
Smriti Mandhana is vice captian of Indian women's team. In June 2018, BCCI named her the Best Women's International Cricketer. ICC declared her the ODI Player of the year in December 2018.