It all depends on the cement and the sand.
A good starting point is one part cement on two or three parts of sand.
But in truth, the issue goes much deeper.
A common misconception is that mortar is glue because it sticks to the brick.
In truth, a mortar is a still liquid or pasty crystal. It needs water to fully crystallize and become concrete, a type of stone.
But the right mixture also depends on the right way to use it.
Before you start, please note that if mortar does not stick properly to the brick, then first make the brick wet. It sticks much better if the brick is wet when the mortar is applied.
The consistency of the mortar is important when working with it, and that is learned by trial and fail. Some say that it is good at the consistency of warm butter or cold yogurt. It should stay put on the brick in your hand, but almost fall off if you move too slowly. It should be soft enough to let the brick be tapped down when placed upon the base, and not so dry that it resists and does not easily cover the whole surface of the brick.
When applying the mortar, one wants the brick fully supported so that the weight from higher rows does not damage it, because the mortar distributes their weight. Some mortar should ooze out between rows, so that one scrapes it off with the trowel.
If you want some definition between bricks, use a tool to create an indentation between bricks, before the morter dries. Check the level of rows with a large level.
Here is a good movie about brick laying.
Mörtel, Mauern, Maurern