Hi fellow readers, I was given a 1-year stay when I visited Chiangmai with a 1-year retirement visa. I have also applied for a multiple entry visa there. I had reported to the Immigration on the 90-days stay.

After a 4 months stay I returned to Singapore, and intend to visit Thailand again this time entry point is Bangkok.

For my coming trip would I be given an 8-months stay in Thailand ( I have a 1-year multiple visa ) and will it be stamped on my passport...do I still need to report the 90-days stay again...Thanks for your advice..

I believe you are saying that you were here in Thailand on a one year retirement based extension and that you have applied/received a re-entry permit for multi entries and then departed.  If you have a re-entry permit stamp in your passport when you return your stay will be valid until your original extension exit time.  You do not add 4 and 8 to come up with a one year visa if that is what you are asking.  The one year is from your first entry and is not changed by exits/returns.  As for the 90 day report that will be due 90 days after you next return to Thailand - your return marks the restart of that timer.