I don’t recommend this for everyone - in fact, it’s rare that a cocktail style reception with no assigned seating really works. But it can!
A couple of things to consider is that if you’re not providing assigned, formal seating for everyone, you still need to provide plenty of OTHER seating - from low-top cocktail tables, to lounge chairs, you still need to have seating for everyone.
DON’T! HAVE A COCKTAIL RECEPTION INSTEAD