While the particle は is commonly known for marking the topic of a sentence, it serves another important purpose: creating contrast. When は follows a number or a counter, it typically emphasizes that quantity as a minimum or an approximation. As a result, it is often translated as 'at least' or 'about'.

• 毎月(まいつき)、本(ほん)を３(さん)冊(さつ)は読(よ)んでいます。
  Every month, I read at least 3 books.
• 昨日(きのう)は８(はち)時間(じかん)は寝(ね)た。
  I slept for at least 8 hours yesterday.

It is common to see くらい (or its voiced version, ぐらい) placed directly before は in this structure. Including it adds a layer of approximation to the statement, making the amount sound less definite and enhancing the 'around' or 'about' nuance.

• 千(せん)円(えん)ぐらいは要(い)るでしょう。
  You'll probably need at least 1,000 yen.
• その映画(えいが)は一(いち)度(ど)くらいは見(み)た方(ほう)がいいよ。
  You should watch that movie at least once.

Although we often translate this as 'at least', the nuance comes directly from the inherent comparative function of は. By marking a specific quantity with は, the speaker is singling it out for focus and implying a comparison with other (usually lesser) quantities. This creates a subtext of 'this amount, (in contrast to some other amount)'. In practice, this feels like saying 'As for quantity (A), in contrast to (B)', even though (B) is almost never stated explicitly.