That really depends - we get about 1,5k tickets a year with about 100mb attachments a year and we run our osTicket on a linux VM with 1 CPU, 2GB RAM and a 20GB disk next to several other web based services without any issues. Since 5 tickets a day sounds pretty much like our tickets' rate I think our config is some kind of orientation for you ;)
You may need more disk space when you get a lot attachments, but when limiting it then it'll take some time to fill 20GB (minus the linux system itself for sure).
