Across languages, plural marking on count nouns typically gives rise to a multiplicity inference, indicating that the noun ranges over sums with a cardinality of 2 or more. Plural marking has also been observed to occur on mass nouns in Greek and a few other languages, giving rise to a parallel abundance inference, indicating that there is a lot of the relevant substance. It has been observed in the literature that both of these inferences disappear in downward-entailing environments, such as when a plural appears in the scope of negation (Tsoulas 2009; Kane et al. 2015). There are two main competing approaches in the literature that aim to account for the described pattern with respect to multiplicity inferences: the ambiguity approach (Farkas & de Swart 2010) and the implicature approach (Sauerland 2003; Spector 2007; Mayr 2015, among others). As discussed in Tieu et al. (2018), while both approaches can account for the upward-versus downward-entailing pattern of multiplicity inferences, they differ in what they predict with respect to the acquisition of these inferences and their relationship with implicatures. Tieu et al. (2014; 2018) investigated multiplicity inferences in English and reported evidence for the implicature approach. In this paper, we first show how the ambiguity approach and the implicature approach to the multiplicity inference can be extended to account for the abundance inference. We then report on an experiment that tests the predictions of the two approaches for multiplicity and abundance inferences in preschool-aged children and adult native speakers of Greek. Our results replicate the patterns reported in Tieu et al. (2014; 2018) for multiplicity inferences, and crucially reveal an analogous pattern for abundance inferences. Adults computed both kinds of inferences more in upward-entailing environments than in downward-entailing ones, and children computed fewer inferences overall than adults did. These results reflect an overall pattern of implicature calculation in line with a unified implicature analysis across the three kinds of inferences. By contrast, we discuss how they pose a challenge for the ambiguity approach.