Is this chair meant to stay outside all year?

It is described as an outdoor dining chair, so it is built for patio and garden use. Cushioned outdoor chairs last longest under a cover or in a shed during harsh winters and heavy rain.

Can I leave it out in the rain?

It is made for outdoor dining, but padded seats hold water and dry slowly. Bring the chair in or cover it when storms are expected to keep the padding in good shape.

Does it stack for storage?

Nothing indicates it stacks. Padded dining chairs of this kind usually sit as fixed places at the table rather than nesting together.

How many chairs should I get for a dining table?

Match the chair count to your table's seating: four for a small square table, six for a standard rectangular one. Order the full set at once so the finish and padding match.