When designing a bot there are two major choices to make at the top:
Will my bot try to make free-form conversation, or will it have a series of menus to navigate? I would recommend a mix of the two, where the greeting prepares the user for what types of questions it is good at, and perhaps a sample menu of the most popular groupings of topics.
For your large list of topics, are most single question-answer pairs? You might want to combine the really common ones into a single dialog and have a menu in there for the user to select the topics. In either case, you will want to build everything in a single PVA bot, currently PVA cannot call another PVA bot as a skill. Once you get into the building of the bot, you will want to enable some of our advanced AI features, like overlap detection, to craft training phrases that will prevent the close topics from triggering incorrectly. You can read more about this here:
https://docs.microsoft.com/en-us/power-virtual-agents/advanced-ai-features