I'm not sure if I understand you correctly. If you reached 100 dimensions limit you cannot do anything with that. You will always have top 100 dimensions available. You can always create table based on this metric, split by dimension (or dimensions -> remember that multiple combination of dimensions will sum to the limit as well), set large amount of time and set limit to more than 100 results. You should see then how many options you have there.
But its not helping me.
The real need is that I created a request attributes to get the SessionId from the header.
I must to count the number of different SessionId that occurred.
For the last 6 Hours for example,
I created a Calculated Metric based on that (on a specific request), but as said, the max showed is 100.
Is there any way to count these SessionId?
The only way I can imagine is to use request attribute as Web request naming rule but this will destroy whole concept of baselines. Dynatrace in general is not BI tool so such tasks (at least so far) are limited.