I haven't been able to find a specific rule in either the rule book or FAQ but the following post from the Steve Jackson Games forum. According to them, the card must explicitly state that the effect stacks. So in your case, the maximum would be +5 to the monsters level.
In my examples, x will always be 5, y will always be 2. A will be
Dwarf and B will be Elf. The Dwarf/Anything or Elf/Anything means
Anything except human. The reason behind this is because of the
Half-breed card that allows you to ignore disadvantages of the single
race you have, as long as you have Half-breed too.
1: +x against A & B = +x if A and/or B.
Monster Card says: +5 against Dwarf & Elf
It means that the monster gets a +5 if you're a Dwarf, Dwarf/Anything,
Dwarf/Elf, Elf/Anything, Elf.
2: +x against A & B and it stacks = +x if only A or only B, +2x if A & B.
Monster Card says: +5 against Dwarf & Elf, and card specifies that it
It means that the monster gets a +5 if you're a Dwarf OR an Elf. If
you're a Dwarf AND an Elf, the monster get +10 in this case.
Dwarf/Anything minus Elf and Elf/Anything minus Dwarf only get +5.
Important note; a Monster getting a bonus against multiple races
usually only get the bonus once. Same for multiple classes, or other
such things, since they're the same source; Bonus vs Race, or vs
Class, or vs whatever. HOWEVER, a combination of those, a Bonus vs
Race AND a Bonus vs Class will always stack, since it's a different
source. Only the monster cards that SPECIFY that their bonus stack for
2 different Races, Classes or Whatever, will have it stack. An example
of such cards would be #3.
3 +x against A, +y against B = +x if only A, +y if only B, +(x+y) if A & B.
Monster Card says: +5 against Dwarf, +2 against Elf. +7 if both.
This one is fairly simple. It means that if you're a Dwarf or
Dwarf/Anything, minus Elf, the monster gets a bonus of +5. If you're
an Elf or Elf/Anything, minus Dwarf, the monster gets a bonus of +2.
If you're Dwarf/Elf, the +5 and +2 stack, and create a +7 bonus for
the monster, hence the "if both".
Negatives can appear instead of those positive Bonuses. Some might
even have both, in which case one would be added and the other would
be substracted. In my examples, it would be something like +5 against
Dwarf, -2 Against Elf. In this case, it doesn't have to specify "if
both" because, while they're from the same source (Race), one's a
Disadvantage (the +5 against Dwarf) and the other's an Advantage (the
-2 against Elf).
The rules (in bold) themselves supposedly come from Steve Jackson (although I can't confirm that), the further explanations are from a forum member.