Hey there Emily - great questions, and thanks for posting this! You sound like you really care about your clients - and I love to see that (as someone with many tattoos that is very loyal to a kind, compassionate tattoo artist!). 

At this time, there is no way to set up rules like this using Workflows - while I really wish there was! With the way Workflows currently work, any invitee that has booked the event you assign the Workflow to will receive that Workflow without exception. This is rare for me to say, but there is truly no viable workaround for this at this time. I apologize! 

I will make sure to mark this as feedback for our product team. I think it would be an amazing feature to have! In the meantime - you might consider creating two separate event types - one for single session tattoos and one for multi-session tattoos. You could then apply a separate follow-up Workflow on each of these event types so that the invitees (clients) for each receive appropriate emails. This might just work!

If you do that - you can also implement routing to make booking smoother and more seamless for your clients. Routing forms let you request information such as industry, company size, specific interests, etc. from website visitors and automatically direct them to a specific scheduling or web page based on their responses. It's great for screening and qualifying sales leads on a website or matching clients or students to the booking page of the right subject-matter expert based on their interests, program, or other criteria. You can read more about setting up routing forms, here (and below)!

I hope this helps!