Epoxy comes in several basic forms.  Table Top Epoxy which is used for depths of up to one inch.  Deep Casting Epoxy which is used for up to four or five inches.  Another type for Deep Pour Epoxy which is used for six to seven inches.  Some artists will try to use the Table Top for a deep pour to achieve "Thermal Cracking" which creates a deep crack in the epoxy which sets the stage for another pour of a contrasting finish which can be quite stunning.  There are many variants and ways to pour the epoxy to get the result you're after.  That is precisely what make the art so interesting.