search the forum and check the docs for latest. It used to be around 8000 elements per polling engine, but there were a lot of variables that affected that.
Now they've got the system doing the calculations and if you look at the "admin" -> polling engines link, it will tell you what % capacity you are at for the number of elements/components and the frequency of checking you do on them.
If you exceed 100% capacity, the system will automatically reduce the polling frequency (doesn't change your settings though) until the load is reduced by admin or adding another polling engine.
If you have different poll frequency settings for different systems/areas of your environment, those settings will stay in-tact but be ignored until issue resolved.
Again - my understanding.