Nothing is stopping you from giving your Animal Companion more Natural Attacks.
Noteworthy, they would be secondary attacks at a -5 until you have and effective Druid level of 9, when the penalty is reduced to -2 by Multiattack.
Select a 1-point evolution other than pounce or reach from those available to a summoner’s eidolon. Your animal companion gains this evolution. The animal companion must conform to any limitations of the evolution. For instance, only an animal companion of an appropriate size and base form can have the mount evolution.
Emphasis mine. Below, I will quote the rules related to limitaions of the Evolution, but the point is that the attack limit is a function of the Eidolon, not the Evolution feature of Eidolons. Because Evolution does not indicate the limitation, it doesn't fall under the limitation on the Evolved Companion feat.
Eidolon Tentacle Evolution:
An eidolon possesses a long, sinuous tentacle, granting it a tentacle attack. This attack is a secondary attack. The tentacle attack deals 1d4 points of damage (1d6 if Large, 1d8 if Huge). This evolution can be selected more than once.
Each eidolon receives a number of evolution points that can be spent to give the eidolon new abilities, powers, and other upgrades. These abilities, called evolutions, can be changed whenever the summoner gains a new level, but they are otherwise set. Some evolutions require that the eidolon have a specific base form or the summoner be of a specific level before they can be chosen. A number of evolutions grant the eidolon additional natural attacks. Natural attacks listed as primary are made using the eidolon’s full base attack bonus and add the eidolon’s Strength modifier on damage rolls. Natural attacks listed as secondary are made using the eidolon’s base attack bonus – 5 and add 1/2 the eidolon’s Strength modifier on damage rolls (if positive). If the eidolon only has a single natural attack, the attack is made using its full base attack bonus and it adds 1-1/2 times its Strength modifier on damage rolls made with that attack, regardless of the attack’s type.
Evolutions are grouped by their cost in evolution points. Evolution points cannot be saved. All of the points must be spent whenever the summoner gains a level. Unless otherwise noted, each evolution can only be selected once.
Eidolon Max Attacks:
This indicates the maximum number of natural attacks that the eidolon is allowed to possess at the given level. If the eidolon is at its maximum, it cannot take evolutions that grant additional natural attacks. This does not include attacks made with weapons.
If the feat was worded limitations of an Eidolon, it would apply... but it's not. And also, Eidolon Max Attacks are based on your Effective Summoner level, so you wouldn't even have one unless the feat indicated to use your effective druid level as your effective summoner level (which it also does not).