2 claws becomes 1 slam for the cost of 1 evolution point. Additional slams would requre an extra pair of limbs and an another evolution point.. In this case the 2 concepts of replacing claws with slams and taking slam additional times are not ideas that are connected in any way.
One slam from replacing both claws from a base form for one point
Additional pair of limbs in order to put additional slam attacks on also using 1 point and 2 limbs per additional slam.
It says in the Slam evolution that you can swap out the base form's claws for slams, but it still costs 1 evolution point to do this.
Does it cost 1 point to swap both claws, as implied by the phrasing? Or does it coat 1 point per slam?
It says that you can swap out the base for claws for a slam attack, not slams. It goes on to state that you need an additional equal number of limbs evolution for to select the slam evolution more than once.
In other words, a set of limbs can get you 2 claws or 1 slam.