After funeral arrangements

Good morning,

I hope someone can offer some advice.
My mother-in-law has had long term dementia, has been in a nursing home and seems to be in her last moments (renal failure, etc.).
My wife is trying to be prepared. We hit a quandary -

We want to provide a sit down luncheon in a local restaurant. The quandary is - how do we invite some at the funeral service without including everyone since that's an unpredictable number?

Has anyone had experience with this. My personal experience in the past - in my family - is that everyone just congregated at someone's home.
But that is not an option here.

Thanks

After funeral arrangements