what is mean if you say so then so be it

I'm only guessing, but it sounds as though the speaker's words have the force of law - at least to that particular listener.

Detail:
There are eight pencils in the pencil cup on my desk.

if you say so, then so be it

means

If that's what you say, then it will be that way.

Context would help. We cannot give better meanings for phrases or clauses if we have the complete sentence.

It means 'AMEN' "End of" It is so' "So be it"
Handshake/contractual-singnature/Deal/DAT IS DA WAY DAT IT IS/Done-bro.