Does anyone know what the maximum number of contacts that can be injected in one go? I know Journey builder is not meant to be a bulk deployment tool but I want to know if there is a limit on it.
When a journey has no entry filter, you can admit up to 2 million contacts per hour. When a journey has an entry filter, you can admit up to 250,000 contacts per hour. Following these general guidelines promotes efficient hourly processing.
Also review 'Journey Builder Best Practices'