A pool has a kind, such as residential or mobile, and a spread across countries. When you ask for a country, your request is given an address from the part of the pool that is in that country. The size of that part decides how often you will see the same address again.
Advertised pool sizes are hard to compare, because every seller counts differently. What matters for your work is whether the pool has enough addresses in the place you need, and whether your target accepts them. The cheap way to find out is to buy a small plan and try it.
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