There isn't a specific maximum number of people that can be considered best practice for a smart list in Marketo. The size of a smart list can vary depending on factors such as your database size, the purpose of the smart list, the frequency of updates, etc.
However, it's generally recommended to keep smart lists as targeted as possible to ensure optimal performance. If a smart list includes a large number of people, it may impact system performance and the efficiency of your campaigns.
It's a good practice to regularly review and refine smart lists to remove inactive or irrelevant leads to maintain their effectiveness. As for the time frame to review smart lists, it depends on your specific marketing goals and the nature of your business. However, it's advisable to review your smart lists periodically to ensure they are up-to-date and aligned with your current marketing strategies. A common approach is to review smart lists on a monthly or quarterly basis, but you can adjust the frequency based on the needs and dynamics of your marketing initiatives.
Ultimately, the ideal number of people and review frequency for smart lists may vary for different organizations, so it's important to find the right balance that suits your specific requirements and resources.