I guess it must depend on the country, but those with proper licensing tend to be restricted to practicing only with people in their jurisdiction.
While weekly is the standard, many therapists will work with you and your needs. Depending on what you seek in relation to ADHD, a monthly session followed by “homework” could be viable
I want to honor that “homework” is not the easiest undertaking for those with ADHD. The assignments should meet you where you are at. The intention is either (a) to help you rewire, (b) have experiences of successes somewhere that you usually don’t or © teach externalization skills to compensate for your wiring. Overarching all this, a good therapist will be able to put words to your experience of ADHD. It’s really helpful in and of itself to have someone grok you