The number of starred expressions is limited by a setting in the file "workspace.json" in your workspace directory. Mine is limited to 10 which seems to be the default (although I didn't fully follow the code for this so I may be wrong). This certainly seems low!
The History is limited by a setting in the same file and is limited to 100 as a default (in this case I can see that in the code).
It is also not great (understatement!) that as soon as you star an expression over 10, then one of your starred expressions is silently removed from your list to make room for the new one.
I'd definitely encourage you to raise a GitHub issue (check if there is already one first) to allow for a larger number of starred expressions and also better management to avoid these being removed without a warning to the user
Owen