dm-best-practices.md: deployment table uses "1~N DM-worker nodes" without defining N

What is affected

dm/dm-best-practices.md (English source, release-8.5 branch and likely master), in the deployment-plan recommendation table:

Deploy 1~N DM-worker nodes according to the number of upstream data sources. Generally, 1 DM-worker node is recommended.

N is used but never defined anywhere in the surrounding text or table. A reader has no way to know what N refers to (e.g., "the number of upstream data sources" is stated as the basis for the count, but it's not explicitly tied back to the N symbol).

Suggested fix

Clarify what N represents, e.g.:

Deploy 1 to N DM-worker nodes, where N is the number of upstream data sources. Generally, 1 DM-worker node is recommended.

Context

Found while reviewing CodeRabbit feedback on a Japanese translation PR (pingcap/docs#23635) — the JA translation faithfully mirrors this same ambiguity from the EN source, so per this repo's translation convention the fix belongs here in the EN source, not in the JA-only PR.

Open dm/dm-best-practices.md on the release-8.5 branch and locate the deployment-plan recommendation table. Clarify that N is the number of upstream data sources, then check the surrounding English text and table for consistent wording. Done means the deployment range is unambiguous without changing the Japanese translation.