Yes, I guess I did obviously miss your original question. As I noted, what you're were asking was unclear, at least to me. Your rephrased questions are a bit clearer. Still not 100% sure what you're asking, but the purpose of VLAN is to avoid the need for additional physical switches to provide multiple LANs. From that perspective, they can be cost effective, but much depends on what your logical and physical LAN topology needs are.
For example, I have the need for two LANs, one to support sales and one to support AR/AP. If they are in separate parts of the building, and I need to populate two wiring closets, where all the users that connect to one closet will be sales and the other closet users all AR/AP, I'll need a switch for each closet, but since all the ports will be in the same LAN for that closet, VLANs don't help. As VLAN switches might be more expensive, they increase the cost.
However, if half the users that will attach to each wiring closet are both sales and AR/AP, to keep them on separate LANs I would need two switches per closet, or one VLAN capable switch per closet. The latter is probably less expensive.
A big advantage of VLANs, then can be reassigned, per port, at will. I.e. if you want to move users around the building, I can reassign their port's LAN by having VLANs.
As to your second question, how many switches do you need for 1,000 PC, again are they all "permanent" connections or are they mostly laptops? If the latter, how many do you need to host concurrently? If they are permanent, presuming you need to provide 1,000 physical ports, the 6513 with twelve 96 port line cards provides 1,104 line ports. I.e. one physical switch might, which answers your revised second question, for the minimum number. But for your original question, how many you need, at the other extreme, if you used 4 port switches, you would need 334 just to provide the edge ports and many more to tie them all together. So the original second question's answer is hundreds to just one.
Also again, physical topology impacts the number of switches needed. One 6513 doesn't help you unless you can connect all 1,000 ports to that physical switch.
