This is one of the most common questions asked about Reiki sessions.
Like a therapy session, a Reiki session is extremely personal. It's therefore difficult to tell someone how many session they'll need to feel better.
Unfortunately, this is not a very helpful answer.
For that reason, I would suggest - in a very directional sense - to think about how long the issues you're feeling have been around. The longer they've been around (i.e. seeded) the more time you may need to break your patterns and have them go away.
You might think of things this way....again using a very general guideline:
A single session will likely help you clear your mind of something recent - such as stress from a bad day or week at work. The calm you feel will often help you feel refreshed.
A group of sessions will often start to change the trajectory of something deeper. Maybe you need to return to the gym, start eating better, feel more confident etc. More than just releasing a recent event, you are looking to begin acting or thinking differently. A group of sessions can often prompt the change needed to begin to go in a different direction.
A longer-term use of sessions helps to cement changes within you. Looking to get rid of feelings for good, or to really feel like you've changed? More sessions will help achieve this as you continue to reinforce the "new" you and say goodbye to your old patterns.
Why does it tend to work this way? The reason, I believe, is not because this is the way Reiki sessions work, but because this is the way YOU work. It takes time to create habits and therefore time to break them. We often hear about the 21-day rule to create a habit - but research also suggests it takes 6-9 months to "defend" these new habits and make sure new ones don't push them out!
Don't worry if it takes time. Feeling better along the way and seeing changes in yourself is often just as pleasurable as hitting some arbitrary goal. When we feel positive, confident, and at peace with ourselves we have wellness...and this doesn't require any particular outcome. It's available whenever we invite it in.
Reiki sessions must be experienced to be understood. Therefore, the best answer comes from actually experiencing a Reiki session or two, and deciding what path you will take.
Every session is helpful in some way, so don't be afraid to begin.
Best of luck to you...