I think it depends on how we define "stealth". If it's nobody knowing, having no clue, you are transsexual, even your group friends and your partner, then I think that's a very very tiny small percentage of trans women. Of course, this level of stealth would involve cutting off everyone who knew from the time before you transitioned and moving way, and I think nowadays, it's virtually impossible due to social networks, everything's connected, so it's very difficult.
But I think if we define "stealth" as interacting with people who don't know about you or even people who knew and you are gendered female, the it's larger pool of people. I think eventually most people can pass, given enough time on HRT, makeup skills, voice work and getting rid of facial hair. Like about 70-80% of women will eventually reach this level if they work it. For some, FFS and other procedures are needed. This is where money counts and it can be really unfair but unfortunately, the more money, the faster you can go stealth.