Open a new post and click 'advanced', then find the attachment button and upload the pictures. From there click the little pulldown button beside the attachment button and insert the pic into the thread.
There is not a limit not on the number of but on the total number of bytes. So if your photos are large straight off the camera, it will stop you from uploading more that whatever the maximum is. Reduce them in size. Anything more that 1600 pixels on the long edge is wasted. !280 is fine. But if you cannot reduce the size of your photos, at least keep using the same thread and not create a new "Topic" as the nomenclature here is.