"The data in UPSID have been used to address the question of the relationship between the size of an inventory and its membership. The total number of consonants in an inventory varies between 6 and 95 with a mean of 22.8. The total number of vowels varies between 3 and 46 with a mean of 8.7. The balance between consonants and vowels within an inventory was calculated by dividing the number of vowels by the number of consonants. The resulting ratio varies between 0.065 and 1.308 with a mean of 0.402. The median value of this vowel ratio is about 0.36; in other words, the typical language has less than half as many vowels as it has consonants. There are two important trends to observe; larger inventories tend to be more consonant-dominated, but there is also a tendency for the absolute number of vowels to be larger in the languages with larger inventories. The first is shown by the fact that the vowel ratio is inversely correlated with the number of consonants in an inventory (r=-0.4, p=0.0001) and the second by the fact that the total of vowels is positively correlated with the consonant total (r=0.38, p=0.0001). However, a large consonant inventory with a small vowel inventory is certainly possible, as, for example, in Haida (700: 46C, 3V), Jaqaru (820: 38C, 3V) or Burushaski (915: 38C, 5V). Small consonant inventories with a large number of vowels seem the least likely to occur (cf. the findings of Hockett 1955), although there is something of an areal/genetic tendency in this direction in New Guinea languages such as Pawaian (612: 10C, 12V), Daribi (616: 13C, 10V) and Fasu (617: 11C, 10V). In these cases a small number of consonants is combined with a contrast of vowel nasality. Despite some aberrant cases, however, there is a general though weak association between overall inventory size and consonant/vowel balance: larger inventories tend to have a greater proportion of consonants."
(Patterns of Sounds, Ian Maddieson)
I made a graphic to show the relation between the number of vowels and consonants in a speech inventory. Each language in the UPSID is represented as a cross in the plot and the gray shading is the density of languages in the vowel-consonant plan.