There are
11 creatures, with this height order: P > P > P > P > P > P > P > P > Z > P > Z.
Reasoning:
The phrase "I am lower" was repeated 20 times:
Plants tell the truth to lower creatures (so they would say "I am higher"), and lie to higher creatures (so they would also say "I am higher"). Zombies lie to lower creatures (saying "I am lower"), and tell the truth to higher creatures, also saying "I am lower". Therefore, saying "I am lower" means being a zombie.
So we know that
zombies spoke 20 times. If there are $z$ zombies and $p$ plants, then each zombie talks to $(z+p-1)$ creatures, so we have
$$z \cdot (z+p-1) = 20 \quad\quad\quad (1)$$
As $p\ge 0$ and $z+p\le 20$, there are three possibilities:
$z=5$, $z=4$, or $z=2$.
The phrase "I am higher and I am a plant" could not be said in 18 occasions:
This phrase can be said by plants to lower creatures (because they tell the truth), or by plants to higher creatures (because they lie), or by zombies to lower creatures (because they lie). So the only case in which clapping is necessary is when a zombie talks to a higher creature.
Let us check the three possibilities for $z$:
If $z=5$, then $z+p-1=4$ (from (1)), so $p=0$. With $5$ zombies and no plants, zombies talk to higher creatures 10 times, so $z$ cannot be $5$.
If $z=4$, then from (1) we need $z+p-1=5$, so $p=2$. We need 18 instances of "zombie talks to higher creature", but even in the best case (P>P>Z>Z>Z>Z), this happens only 14 times. So $z$ cannot be 4.
If $z=2$, from (1) we have $z+p-1=10$, so $p=9$. We need 18 instances of "zombie talks to higher creature". As both zombies cannot be in the same position, the only possibility is that one zombie talks to 10 higher creatures, and the other one talks to 8 higher creatures. This leads to the final result:
P > P > P > P > P > P > P > P > Z > P > Z
