You are using the terms "model" and "theory" in an idiosyncractic way.

In model theory, a *model* is a first-order structure, that is, a set with some functions, relations and perhaps distinguished elements, called constants. A *theory*, in contrast, is a collection of assertions, a set of sentences in this language. A given theory, which can be thought of as a set of axioms in the sense that you mentioned, can give rise to many models. And indeed, the Lowenheim-Skolem theorem says that if a theory has an infinite model, then it has infinite models of arbitrarily large cardinality. (Thus, except in trivial cases one cannot uniquely specify a model by giving "some (countable) number of axioms" as you said, since the same axioms will have models of many different sizes.)

Suppose that M is a model in a language of size κ, meaning that the language has κ many possible assertions. In this case, since any given assertion is either true in M or its negation is true in M, the complete theory of M, that is, the set Th(M) consisting of all sentences true in M, will also have size κ. Any subset S of Th(M) will also be true in M, of course. Thus, there are 2^{κ} many theories true in M. For example, if the language has countably many symbols in it, then any given model in this language will satisfy continuum 2^{ω} many theories.

But this answer counts theories as different, when they are different merely as sets of sentences, even when these theories have the same models. But for the purposes of counting theories, it may be more sensible to use another common definition of *theory*, which is a set of sentences closed under consequence. This amounts to identifying theories that have the same models.

With this second understanding of theory, the answer is a little more subtle. In the empty language, for example, every model is just a naked set, with no structure. There are exactly countably many countable models in this language: one of each finite size and one countably infinite model. If φ_{n} is the assertion that there are exactly n objects, then for any set A of natural numbers, we may form the theory T_{A}, which asserts that ¬φ_{n} for each n in A. These theories are all inequivalent, and all true in any infinite model. If M is any model, then there are continuum many theories T_{A} that are true in M.

This shows that in fact every model M, in any language, satisfies at least continuum many deductively closed theories.

If the language is larger, with uncountable size κ, then either there are uncountably many relation symbols, uncountably many function symbols or uncountably many constant symbols. In each case, it is a fun exercise to form 2^{κ} many inequivalent theories T in the language. Given any model M, let σ be any sentence false in M. For any theory T containing σ, we may form the theory T' = { σ implies φ | φ in T }. This theory is true in M, since σ is false in M. Thus, by counting theories in this manner, one can show that there are 2^{κ} many inequivalent theories true in M.