One is probably enough for pretty much all cases, though I like to hold onto an extra of non-Season 1 Heroes, because they’re harder to come by, even for 3*.
I have one at +18, and a second maxed, who used to get decently frequent usage, typically along with a duplicate of Proteus and Gadeirus.
Further upgrades to my roster have made him see battle pretty infrequently.
My +18 one gets used plenty, but my duplicate maxed one not as much now (see Triton above).
I have one at +18, who’s had a ton of usage.
I’ve never felt a need for a second.
I have one maxed, who hardly ever sees usage, although that might change.
I can’t really see using a second often, unless I made a point of it to change up Raiding for fun.
I have 3 maxed, one of which is at +18.
The first one sees a ton of usage. The second sees a decent amount of usage.
The third sees rare usage, but I’m glad to have him.
I came close to maxing a fourth, but have held off.
Hero Academy Disclaimer
A reminder that we have no idea what Hero Academy will do, or how, but it could perhaps possibly entail some use for unwanted heroes.
So eat extras according to your tolerance for risk for regret later vs. them being in the way now — that equation balances out differently for each person.