I also would like to be able to customize a number. Can’t be that hard could it?

Have heard NO rumors or chatter. After checking each ball in the inventory, it appears that you cannot get a custom number on any of the Titleist yellow golf balls, only (1,2,3,4) which is standard. No high nos. either. So what you read might be true. Could be a demand issue. I do not see many yellow Titleist when I play, hardly ever find a premium one.

Correct. If you tour Ball Plant 3, you will see that numbers are added in the batch assembly process as opposed to adding customization on the sides which is done after final ball production. Each high number must be done in a large batch which means they must hold supply until that batch of a particular number sells out. While yellow is popular enough to stick around, it's not creating the very high volume needed for these extra numbers. Maybe some day but until then, I am just glad we continue to have the yellow option. I do not play them regularly but like to have a few on hand for rainy or foggy days.

The numbers are applied in the same step as the logo. The plant is making millions of balls. Not a high percentage are yellow. When the run the 5-99 numbers, they are making thousands at a time. Demand is strong enough that the quantity needed is reasonably stable. Once you get into the yellow balls the number of customization number sales won’t be as predictable. All the balls denoted as custom numbers go to a different processing location than the standard 1-4 boxes.
It is the chicken and egg story. If yellow balls had a higher demand in general, then customization would be more feasible. Unlike tennis, yellow just isn’t catching on as fast. Did note this past weekend that yellow is becoming more common on the Champions tour.

Sure, I would love to have my own personal 2digit # in yellow. If you had the option for 1-8, would that be better? Having waited 3 releases for any yellow Pro-V, my need has been met although I still want for a custom number.