Per-request, sticky and hybrid rotation, with the failure modes of each.
The short version
Figures in this piece come from providers' published pricing and documentation. Where a provider publishes no number, we say so rather than estimate one.
The detail
Every listing in the directory uses the same layout — pricing tiers, pool size, targeting options and features side by side. That is what lets you compare providers whose pricing models have almost nothing in common.
What this means for you
Start from your workload shape, not from a provider. Once you know your tolerance for retries and your concurrency ceiling, the shortlist usually collapses to two or three names.