Each of the Helium 10 plans provide customers with a fixed number of Uses on most of the Helium 10 tools.
You can view how many Uses you get on each tool per plan by clicking on the Plan Details button found at the top right of your Helium 10 member dashboard.
The Uses that your plan provides on a monthly basis do not stack up if unutilized, but will just replenish at the start of every billing cycle.
Choosing a plan will be dependent upon how much you would need to use each tool. You would also need to consider the size of your business as well as the amount of work that you need to do when creating, optimizing and monitoring your listings.
If you feel that the plan the you are currently subscribed in is too large for what you need or perhaps you need more, you can always upgrade and downgrade any time you desire. (The system may limit the number of plan changes made in any one given billing cycle.)