A number is a mathematical object used to count, measure, and label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. A written symbol like "5" that represents a number is called a numeral.
When the authentic teaching from the Prophet (SAWA) or Ahlul Bayt (AS) tells us to repeat reciting something, it means that we need to keep on focusing and concentrating on the meanings of it again and again because it will elevate our spiritual benefits as well as increase our reward and blessings. All the teachings of Islam are based on our benefit. Similarly, repeating certain Dhikr or Du'a is for the same aim of benefiting us through repeating which makes us concentrate more on the great meanings of what we recite.
There is no specific significance to the number 18 in Islam, and it is not connected to maturity in Islam, from what we can see from our mainstream Islamic tradition.
And Allah knows best.
We find in many Quranic verses the mention of number forty:
and Al-Ahqaaf 15.
We also find in many Hadeeths the mention of the number forty.Forty years in Quran is the peak of the development of the human being (Al-Ahqaaf 15).
Forty years was the period of the punishment of Bani Isreal (Al-Ma'edah 26).
Forty nights was the period of the absence of Musa from his community to test them.
Forty days is the period after the martyrdom of Imam Husain (AS) when his son Imam Sajjad (AS) and his family returned back to visit him in Karbala. That is the time of Ziyarat al-Arba'een attended by millions every year.
It means that this number has important indication for reaching to maturity or completing the aim which Allah (SWT) wants. Details of the secrets of this number is best known to Allah (SWT) and His messenger and Ahlul Bayt (AS).