How do I get my label retrieval to show more than 20 posts?
The "Show" setting, in Settings - Formatting (now, "Posts, comments and sharing") 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.
Since main page in dynamic view is not paginated - it displays continuously, as you scroll down the page - the "Show" setting will not apply there, either.
Fortunately, Jump Break appears to shorten both the "archive" (date organised archive display) and "label" (label organised 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.