Given a number $n$ and a big rectangle of size $a\times b$, you want to find a rectangle size $x\times y$ such that $n$ such rectangles fit exactly into the big rectangle. A simple method to achieve this, is to find a factorization $n=m\cdot k$ of $n$ and let $x=\frac am$, $y=\frac bk$. This way it is possible to lay the small rectangles in a regular $m$ by $k$ pattern and thus fill the big rectangle.
For example, if $a=320$, $b=480$ as in your example and $n=100$, one might choose the factorization $n=10\cdot 10$ and thus arrange $10$ by $10$ rectangles of size $32\times 48$. Or we might factor $n=5\cdot20$ and arrange $5$ by $20$ rectangles of size $64\times 24$. Each factorization $100=1\cdot 100=2\cdot 50=4\cdot 25=5\cdot 20=10\cdot 10=20\cdot5=25\cdot 4=50\cdot 2=100\cdot 1$ gives rise to a different solution.
An interesting question arises: If we allow the small rectangles to be rotated by $90^\circ$ and thus use "mixed" patterns, could solutions with other sizes than those found by the method above come up?
The somewhat surprising (partial) answer is: One of the fractions $\frac ax$, $\frac bx$ must be an integer, and one of the fractions $\frac ay$, $\frac by$ must be an integer (this can be shown using a clever integration trick). In special cases (e.g. if the given rectangle is a square, $a=b$), this already shows that $x\times y$ is among the solutions found with the factorization method described above.