our customer has requirement of AVEVA Historian 2023 R2, Standard 12,000 Tag and AVEVA Historian Client Desktop 2023 R2, Concurrent User Single. Now customer has query 1)How many tags can be deployed for trend pens on a single window?? 2)Any limitations

Not sure I understand but you’re a limited by the resolution factor. You shouldn’t try to make the maximum but instead try to better understand your client requirements and perhaps discuss with him about the fact that if you’re loading your screen with unnecessary information, it will have a bad impact on cognitive comprehension and decision making of the operator.

by the way I’ve seen trend managing something around 10 pens at one time without issue (stacked) - need to try it again 

There is a max of 30 tags in Historian Client, but the practical limit is usually lower, as Loris mentioned: too hard to interpret the data if you get more than ~10 pens.