A thumbnail is seen at about the size of a postage stamp before anyone decides whether to click. Everything about making one follows from that.
The mistake everyone makes is judging a thumbnail at full size. Scale your finished image down to about two hundred pixels wide and look at it there. If you cannot read the text or tell what the picture is, neither can anyone scrolling.
Three or four words is the ceiling. The title is already next to your thumbnail, so repeating it wastes the space. Text on a thumbnail should add something the title does not.
Faces and high contrast do well because they survive being tiny. Fine detail, small text and busy backgrounds do not.
A thumbnail is seen very small, often smaller than a postage stamp, so the words have to be few and big. Three or four words at the most, in high contrast, work better than a full title. Raise the size towards the top of its range and keep the outline switched on.
Crop your picture to a 16 to 9 shape first with Crop & rotate so the text is placed on the frame as it will be seen. Then look at the result shrunk right down. If you cannot read it at that size, make the text larger or shorter.
1280 by 720 pixels is the standard, and any 16 by 9 image scales to it cleanly.
Most platforms cap thumbnails at a couple of megabytes. Compress if yours is larger.
No.
Three or four at the most. Anything longer becomes unreadable once the thumbnail is shown at its small size.