Hi Mark,
Thanks for your interest. The answer to your question is directly dependent on the maps and data you intend to consume with Explorer. If you can provide more details on your data and service configuration, we should be able to provide some general estimates- but we won't be able to guarantee usage stats.
Do you deliver your data through map services or feature services?
How many layers would be in the webmap?
What geometry types would you be using? And how many features would you typically have per layer?
What is the coverage of your data? City, State, National?
Regards,
Nick