From memory-
AP-225 can associate 255 clients. Best practices say 150 clients maximum. Default config only allows 80 associations. 80 on a single AP would perform poorly in most cases.
Number of associations is not a valueable metric for HD design. You have to think about the use case. Sure an AP may allow hundreds of associations, but imagine if all clients were streaming HD video. That design would perform poorly.
No easy answer to this question. You'll have to get familiar with the Aruba design guides.