The amount of X's that can fit inside Y?

Normally when we work out how many of X can fit into Y we divide Y by X. Is this the right approach when maybe X can’t be perfectly stacked flush next to another X such as with two circles?

If Y was a square and X is a circle should we divide Y by two times the radius of X?

Now imagine Y is a circle and X are also circles what’s the best approach at working out how many X’s fit into Y.

Are there some rules to follow when the shapes become more random?