Munchkin: card attributes in the event of helping another player
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 &amp; B = +x if A and/or B.
  
  Monster Card says: +5 against Dwarf &amp; 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 &amp; B and it stacks = +x if only A or only B, +2x if A &amp; B.
  
  Monster Card says: +5 against Dwarf &amp; Elf, and card specifies that it
  stacks.
  
  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 &amp; 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.