I think the 'then' would be helpful, as you can't mark 'can' for the future (at least, not in standard English - I think there are some dialects (Geordie?) that accept 'will can').
But adding 'then' at the end to clarify the future sense of 'can' doesn't sound fluent, to my ear. I think most native speakers would use a to-infinitive: '...to let more people/allow more people to sit down' or even something like 'to accommodate a bigger audience'.
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