Full Idea
Quine takes 'existence' to be univocal, with a single ontology for his entire 'web of belief'.
Clarification
'Univocal' means having only one meaning
Gist of Idea
For Quine, there is only one way to exist
Source
report of Willard Quine (On What There Is [1948]) by Stewart Shapiro - Philosophy of Mathematics 4.9
Book Reference
Shapiro,Stewart: 'Philosophy of Mathematics:structure and ontology' [OUP 1997], p.141
A Reaction
Thus, there can be no 'different way of existing' (such as 'subsisting') for abstract objects such as those of mathematics. I presume that Quine's low-key physicalism is behind this.