Anesthesia fellowships are generally not very competitive. Especially CC and Cardiac. They are work-horse fellowships. Peds is a little more competitive and pain is the most competitive (I would argue) but any candidate from a top 50ish program can get into any fellowship.
Now that being said, getting into a PARTICULAR program can be competitive. Meaning, "I want to stay in Boston for pain" or "I want to do Cardiac in LA" that might be hard, there might be 200 people competing for 20 spots, many spots go to internal candidates, people with personal connections, etc..
Any candidate from a Top 50ish program can get into any fellowship. Do well in residency, apply broadly. After you have gotten into medical school and residency, the anesthesia fellowship process is a complete joke. It may be a very random and disorganized process, but the doors to ANY fellowship are always open to all candidates (who have no major red flags)
How could this be? Well you are giving up 300k of income to do this year that honestly will probably NOT be required for your future job. It may enrich your experience and satisfy your academic curiosity and add to your overall expertise, not saying fellowships are useless, but definitely not REQUIRED for the vast majority of good and exciting and high intensity jobs out there
In the end, if you decide to do a fellowship apply to lots of programs (40-50+). Go on about 10-15 interviews. And if possible try to stay at your home institution for simplicity reasons