There is cases where a trigger may have more than 9 items in the calculation.

To be able to list all of the items last values in the trigger description (used for alerting) and with the limitation of <1-9> this is not possible.

Could we have this increased or make in work in a way if there is a index macro it accepts any number?

If performance is an issue that this was not initially done, this would be up to the Zabbix admin managing the instance whether they do this or not, however the functionality would still be there.