can you just stack a cake without putting sticks in the cake to stack it , or do you have to put sticks in the cake in order to stack it ?

If you stack a cake, the bottom tier needs something in it-dowels, straws, etc.-to support the weight of a top tier or tiers.
If you are just making a two layer cake, no support needed.

I've never had a problem with a 2 layer cake, however, any more tiers than that would require dowels or something.

The rule is that you need support (SPS, dowels and cardboard, or some system) for every 4" of vertical height.