We’ve never had more than one bartender and one food host. You have very little interaction with the food hosts, but they are always there replenishing the food in the buffet. The level of interaction you’ll have with the bartender depends on how much you use his/her services. We request a few cocktails a night, so they often get special bottles or ingredients for us, and we end up interacting a lot with our bartenders.
As far as tips go, we tip everyone on the last day of the cruise, but we are not Americans so tipping is not natural for us, as is use of cash. We get extra envelopes for the food host and bartender. Concierge hosts tips are pooled, so there‘s no need to tip one specific host. We usually tip 20$ per night for the bartender, so 150$ for a 7-nights cruise, around 50$ for the food host, and the amount for the concierge hosts vary depending on how much service we required from them and our level of interaction.
Concierge tipping is all over the place on these boards and on FB. We just do what we feel is justified.