Knowing when to hire your first BD person can feel like a cart before the horse situation. I'm a firm believer in having two things that trigger hiring a BD person.
A CRM in place AND your current team uses it.
Multiple people are trying to do BD tasks and there is too much to handle.
Having a CRM system in place will allow the BD person to feel supported and gives them a roadmap of how to bring in other people from your firm into client discussions.
Once you have multiple seller-doers conducting BD tasks and it feels messy, that's a key indicator that you need to bring someone in that can coordinate the growth effort.
However, you can't do No. 2 without having the CRM in place.