Where to find these values: Meta Events Manager → your pixel → Settings tab. Pixel ID is at the top. Conversions API access token is under "Set up manually" → "Generate access token".
Test Event Code is optional. In Events Manager → Test Events tab, copy the code shown there (e.g. TEST12345) so test events route there instead of production.
Multi-pixel use cases: main pixel + backup, or one pixel per ad campaign. Each enabled pixel receives every event independently — Meta dedups against client-side fbq via event_id.