1 of each if you want any respect, running 3 of the same class will mean that you won’t be taken seriously

I've always had one of each purely for the variety. Makes some of the grind less grindy by playing each Iron Banner with a different character with different subclasses, for example. I levelled up my Hunter and Warlock by passing on 950 weapons from my Titan, leaving all their armour at 750. I then went into collections and purchased the blue 'Levelling' armour which came in at 800+ until it levelled out at about 850. Then I cashed in the Season armour on each of those 2 characters, collected clan engrams, etc and reached about 940 on both without playing any of the Season 8 content. From there I'm playing things like Iron Banner for Pinnacle gear. I still main my Titan as I don't have time each week to play all 3 but it's handy to have 2 backups at roughly the same level for when friends/clan want help with something I've already completed. In Destiny 1 I levelled up my Hunter solely by playing Crucible as I just didn't fancy doing the campaign a third time.