Number + も

The adverbial particle も is often used for labeling (A) as part of a category, when it would not usually be considered part of that group. In this way, it emphasizes that (A)'s inclusion is something that is surprising/unexpected in some way.
When も is used after a specific number + counter, it shows that the number itself is surprising. This can be when the actual number is lower, or higher than what was expected. Due to this, も can translate as 'as much as (A)', or 'not even (A)', and other similar phrases in this expression.

• 12時間(じかん)も仕事(しごと)をしたから疲(つか)れた。
  I worked as long as 12 hours, so I am tired.
• その携帯(けいたい)20万円(まんえん)もしたの？！
  That cellphone cost you as much as 200,000 yen?!
• 私(わたし)は一回(いっかい)も地下鉄(ちかてつ)に乗(の)ったことが無(な)い。
  I have never taken the subway, not even once.

At first glance, this may seem different than も's translation as 'too' or 'also', but in those cases, も is also just pointing out the something unexpected is part or whatever group was initially highlighted.

• 私(わたし)も行(い)きたい。
  I want to go too.

The above sentence just shows that the (B) speaker is unexpectedly putting themselves in the group that the (A) speaker initially only considered themselves to be a part of.