I am wondering, is there a suggested optimum number or posts per thread in miniBB? f

I think it's totally up to you. I would say between 10 and 40 will be fine.

I have one with over 1000 posts and wondering if it should be closed.

Oh, sorry - I initially thought you meant how many messages per page.

I know there are threads with a few thousand posts in one thread so 1000 is not too many :)

Thanks for the tip. Glad to know it's "safe" :)

It does depend only on how comfortable your users will feel browsing such large topics ;-) Technically there are no limits; but I suppose as soon the topics gets to the amount which becomes hard to browse, it's better to close it and start kind of 'topic pt.2' regarding the same subject.