BIG-LTM-1500 capacity and support

we have 3 sites, each is using two load balancers BIG-LTM-1500. we have below two questions: 1- what is the dimensioning limit (number of request) thaet this LB can handle and how to measure it? 2- is it configurable that in case two LB of site 1 for example are down, that LB of site 2 handle traffic for site1 and site2 and forward the requests correctly to each site?