You cannot do this directly in the lookup. You can create a conditional rollup for each select option that counts the number of times that particular option appears. Set the rollup condition to include only records where the select is a particular choice. Use the rollup formula COUNTALL(values)
Then you could have a different formula field that combines the values of those rollup fields.
CONCATENATE(
"- " & {rollup for urgent} & " urgent ticket(s)\n",
"- " & {rollup for high priority} & " high priority ticket(s)\n",
"- " & {rollup for medium} & " medium ticket(s)\n",
"- " & {rollup for low} & " low ticket(s)"
)