How do you convert count per second to becquerel?

1 Curie is defined as 3.7 x 1010 disintegrations per second, so a millicurie will be 3.7 x 107 dps (or Bequerels as it is now called). This is a total quantity, if you have a piece of material which produces this number of Bequerel it is said to be 1 millicurie of that material.

In terms of counts per second perceived by a detector, it depends very much on the design of the detector. For example what is the solid angle subtended by the detector, what type of radiation is it detecting, what is its efficiency in detecting and counting the particles emitted? So there is no simple answer to the question.