Theoretically the limit is 2048, but in practice you can go well over that and everthing works fine, except that yuo get a nag screen every 5 minutes or so. I've been up over 2800 without any problems.
Somebody commented in another Thread that there was once a licence for 8096 Tags in an old Siemens price list, so that would suggest that there should be no inbuilt S/W problems if you do need to exceed 2048.
Just a question of whether your customer can live with the nag screen in that case.