Hello Martina Long,
Something must have gone wrong with the posts since my previous replay was for other support thread.
Regarding your question, the Items.Count property should return the correct number of items in the listbox, we are not aware of any problems with that functionality. Can you provide us with more details regarding your scenario, perhaps some sample code & markup. I have tried the following code and it seems to work:
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items.