How do I get my label retrieval to show more than 20 posts?
The "Show" setting, in Settings - Formatting, applies to the main page display size, only. Archive and Label retrievals are limited to 20 posts / page, as a fixed maximum setting. Archive and label retrievals are also subject to auto pagination limits.
Fortunately, Jump Break appears to shorten both the "archive" (label archive display) and "label" (label archive display) page lengths - just as it does the "main page" length. If you need to bring your archive or label page display size under control, using Jump Break is the only choice provided.