If both parents have b+ blood type, can their child have a different blood group?and if the parents have 2 children, can only one child differ?

Yes to both: If both parents are B+, any of their children can be either B+ or O+. Blood type is determined by the presence of either an A or B antigen, regardless of amount., with inheritance from both parents. B+ parents can be either BB or OB,. If both parents are OB, the child can be B+ (inherit a B from either parent) or be O (inherit an O from either)