You want to connect 250 clients to one sector or to one tower (with many sectors)?
I would suggest 8 sectors. 250/8~=30 customers per radio interface. That's possible, with NV2 can be quite efficient.
I once managed a tower with 8 sectors + 5 point-to-point links to other towers. The spectrum was also occupied (by other ISP's), but the radios worked properly. I used a couple of RB433's and a RB800 with typical 802.11a 100mW cards.
Do you suggest this number (30) due to your experience? Why would happen if I add more customers?
for example 40 customers is too high?
thanks again