If the evolutionist has more than one natural weapon, he must decide which is the primary and which are the secondary ones upon taking the mutations, that choice can be changed whenever the evolutionist gains new mutations. If the evolutionist has multiple sets of the same natural weapon, only one group (one bite, one pair of claws, etc) can be the primary.
The evolutionist can have up to one bite and gore per head, one tail, four tentacles, and up to one claw or slam per arm (any combination of these, but no more than one of each per limb), if he chooses to, he may evolve only one claw, tentacle or slam instead of a pair, but each still costs a mutation.
Further Mutations: Each time this mutation is taken the evolutionist can chose to improve the damage of one group of existing natural weapon by one step or gain a new natural weapon from the table. This mutation can be taken up to once per two mutator levels.

Is this saying you can increase a natural weapon an additional 9 times if you focus on it up to lvl 20? If so how's that even work? Is this a size increase? If so 9 times isn't even possible by the chart in most cases? I'm just really confused here.

Or is the 1st bolded line saying one increase is the limit?

Some clarification would be good, as is we are getting tail attacks for example that can do 12d8 damage if we go as far as the chart can go. Is it going by an altered progression?