Monsters at the start usually have 2 or three skills.
examples.
Tier 1 Goblin usually have 2-3 skills and/or the first stage of skill can rival F - Rookie E rank adventurers
Tier 2 Goblin elite usually have 3-5 skills and/or 2nd stage of skill can rival E to rookie D rank adventurers
Tier 3 Hobgoblin 4-6 skills and/or 3rd stage of skill can rival D - Rookie C rank adventurers
Tier 4 High Hobgoblin 5-7 skills and/or 4th stage of skill can rival Veteran C - Early B rank adventurers
Tier 5 Hobgoblin King 6-8 skills and/or 5th stage of skill can rival Veteran B rank adventurers
Tier 6 Monster Generals (think of them as leaders that serve under the next tier Albeit unwillingly) 6-9 skills and/or 6th stage of skill can rival A to S rank adventurers.
Tier 7 Monster lords 10 - ?? skills with the 7th stage of a skill can be compared to Champions or black cardholders which is the last rank of adventurers which is ss rank who often had a lot of different skills.
----------
Now on to some other things related to monsters.
The evolution tree earlier is a common way for a goblin to rise in power if someone puts an effort into gaining another skill they can surpass others with time and effort.
Monsters don't try that unless they had a certain habit that made them repeat an action according to their chosen race.
Their instincts and nature are what drives them to never rise in power, however, if there was an outside influence, like being enslaved, tortured, left for dead, the death of loved ones, they'll seek power to deliver their revenge making them not very different from the other races when it comes to this part of nature.
The different races, that serve under the gods, of the world can also gain skills, however, thanks to the restriction laid down by the gods they had to put in 10X the effort to receive a new one which consumed a lot of time to which many didn't have.
The reason being, when an individual gains a certain amount of power they can join the ranks of gods which will make things harder for the current gods that need the faith power given by their believers
The evolution of a monster is related to what their skills are and what level it is, so unless they leveled their skills to a required limit, there was no chance of them evolving something that many monsters didn't know but few instinctively understood.
Subevolution is where the monster experiences a 'race change,' however, they retain their old racial abilities which would evolve with them at the same time.
The more 'subevolutions' undergone by monsters the more skills they gain and stronger they become until the third tier of monsters a single monster can only have two 'subevolutions' before going with the flow of nature.
With every change a monster undergoes, they could gain 1-3 more skills according to how they lived before undergoing the evolution.
P.s. Mark's starting race tends to be terrible assassins in their first tier, not warriors that blindly charge forth, and the more skills one has the stronger they can become allowing them to handle higher-tiered monsters~