The activity is greater the greater the radioactive constant ( l ) and the number of nuclei present.
The initial activity of an isotpoe is greater at the beginning, provided that the isotope formed is not also radioactive in which case we have a  radioactive series.

In this visual you can see in a qualitative manner how the activity of a radioactive sample decreases as time passes.

The activity of a substance depends on the quantity of the sample and it units are: the Curie and the Rutherford:
1 Curie is the activity which corresponds to 1 gram of radium and produces 3.7.10¹⁰ decay/second.
1 Rutherford corresponds to 10 ⁶ decay/second .