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According to Amazon JP, both the first and second OPM volumes are 200 pages long. That's the official number, and it's probably the same source that the TG wiki is using.
And actually, even with Murata's fuckhuge chapters the volumes don't become longer, the chapters just get split between two volumes, or at least so far.
Uh, the one you found on Amazon in English isn't going to be ONE's webcomic. That's just a webcomic that's online for free. They're all written by ONE because ONE is the one who writes all of it. You aren't going to find OPM not written by ONE.
This is what you're looking for.
Viz is releasing it in English. You can probably find it in the manga section of your local bookstore or you can find it on online retailers like amazon here.
https://www.amazon.in/One-Punch-Man-Vol-1-ONE/dp/1421585642/ref=asc_df_1421585642/?tag=googleshopdes-21&linkCode=df0&hvadid=396989343825&hvpos=&hvnetw=g&hvrand=8616641196189339072&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9302878&hvtargid=pla-404766145919&psc=1&ext_vrnc=hiis link pe jake dekh author ki taraf se hi hai
Here is the Amazon link for One Punch Man vol 1 manga. The cheapest price is $16 dollars with shipping. https://www.amazon.com/dp/1421585642/ref=cm_sw_r_apan_glt_fabc_T2P76Q1QVKYFN591REYZ
Amazon sells them but if you want to get it in person I get mine from Chapters/Indigo but I'm not sure if you live near one.
Amazon seems pretty cheap
One-Punch Man 1 price matched on Amazon, if no one's bought it yet...
Amazon.com? Probably English.
Amazon.co.jp? Probably Japanese.
You should be able to tell which one you are buying by the text in the bottom left corner of the books in their images.