We present heuristic asymptotic formulae for the number of isogeny classes of pairing-friendly abelian varieties (PFAVs) over prime fields, generalising previous work of John Boxall on elliptic curves. By enumerating Weil numbers satisfying "pairing-friendly" conditions we were able to count the number of PFAVs and compare with the heuristic value. In all the examples we computed, the results show good agreement with the heuristics.