Dear Student,
Atleast means minimum number, while atmost means maximum number.
For example:
If number of events are atleast 3 and atmost 8, then minimum number of events are 3 i.e. It could be greater than or equal to 3 but not less than 3. and maximum number of events are 8 i.e., It could be less than or equal to 8 but not greater than 8.