You want to allow enough connections to support your load, while not allowing enough concurrent connections that your system is not able to respond quickly, thus preventing a DOS attack. The max concurrency number is equal to MAX_PROCS * MAX_CONNS, defaults are 10 and 20 respectively.
Once you know the max concurrency rate for your given configuration this will help you determine the number of systems required to support the load at your site.
MAX_LIFE_CONNS should be adjusted if you are using Direct LDAP Mode, every site should be using Direct LDAP Mode, if at all possible. This should be at least twice, and probably four times ALIAS_ENTRY_CACHE_SIZE , to take advantage of the cache. See Section 8.11
To find an optimum value for MAX_HANDOFFS (Check here) (default value is 5), you need to figure out how long it takes for a connection to be ACCEPTED (you can look in Dispatcher debug logs for that), and then estimate how many connections you expect the Dispatcher to receive during that time period. If the time between the connection coming into the Dispatcher and being ACCEPTED by the worker process is 0.5 seconds, and the system should be able to handle 30 connections/second, then MAX_HANDOFFS would need to be set to at least 15 (30 conns/second * 0.5 seconds).