How many lessons per day you want to take will depend on how much free time you have; how big of a workload you can handle. The more lessons you take, the more reviews you will have.
If you want to go at maximum speed, you need to take 20-25 lessons per day (there are some levels where you can go even faster, but that’s not something you need to worry about until later).
However going at full speed takes a lot of time, and you can easily end up doing 200+ reviews everyday at that speed, which can get overwhelming and lead to burn out. For a more reasonable but still steady pace, you can take 10-15 lessons.
Basically the idea here is that you can use the number of lessons to adjust your pacing. If you are feeling overwhelmed, reduce your number of daily lessons (or stop them altogether until the number of reviews gets under control). On the other hand, if you feel you can handle more reviews, take more lessons.
There’s no need to take all lessons as soon as they are available (but of course, you can if you have the time and energy to handle the workload spike this will create).
Now, the title of your post also mentions reviews. It is usually suggested that you do at least two review sessions per day. This is because the way that wanikani works, new lessons you take will be reviewed at the interval of 4 hours, and then 8 hours.
So for example, you can build a schedule where you wake up at 8 am, do lessons, then do your first set of reviews at 12 pm, and your second set at 8 pm, which matches those intervals. This is recommended because these intervals have been adjusted to be in the general period of time where you will be close to forgetting them, and reviewing them at that time will help with memorization.
This is a general guideline. You will need to adjust depending on your own circumstances.