If “Rose Tentacles” gains extra attacks at the start of your Battle Phase, but is then flipped face-down or temporarily leaves the field before it attacks, it can only attack once during that Battle Phase after it returns or is flipped face-up again.
The number of attacks that “Rose Tentacles” can perform is determined at the start of the Battle Phase. If the number of Plants changes, the number of attacks “Rose Tentacles” can perform doesn’t change.
The number of times "Rose Tentacles" can attack is determined at the beginning of the Battle Phase. Afterwards, if the number of Plant-Type monsters your opponent controls changes, then it does not change the number of times that "Rose Tentacles" can attack.