Colonization of a new smooth surface of a tooth will occur by S. mutans when its  level in the oral cavity reaches:
Levels of S.mutans greater than 105 are indicative of an acceptable cariogenic challenge because colonization does not occur until the level of S. mutans reaches 4.5×104/ml for smooth surface and 103/ml of occlusal ﬁssures. 
Several clinical studies have corroborated the ﬁnding that the percentage of S.mutans in saliva increases signiﬁcantly as a function of the DMFS score. 
There is thus substantial evidence for associating high S.mutans counts with caries activity.

Shafers textbook of Oral Pathology – page 468
>105/ml
4.5 x 104/ml
3.5 x 103/ml
5.5 x 102/ml