my heuristic is that channels should be 1000000 minimum and one should have a minimum of 2 of them, one inbound, the other outbound, it really depends on how much money goes in and out on a monthly basis but you don't have to keep them even much better than 25% filled at any given time, but it's really worth having that extra capacity in case some people decide to send you or you need to receive more than 100ksat several times before you are moving it elsewhere