Is it better to say amin or suma amin after you completed your dua? at the end of salat

and can you say it more than once? like first you ask for what you want, and then say amin/sum’amin and then rabana zalamna… etc and then again say amin/sum’amin??

Aameen means 'May Allaah it happens like that (what is being asked in dua).

Aameen is to be said by those who listen a dua being asked by other than himself. For example if I ask a dua " May Allaah bless you" then the person who listens this dua may say Aameen as well as Summa Aameen.

The person who himself ask any dua is not required to say Aameen at end of dua.

بِسْمِ اللَّهِ الرَّحْمَـٰنِ الرَّحِيمِ

Assalaamu’Alaikum WarahmatUllahe Wabarakatuho

The Meaning of Aameen is as my brother said in above answer. And about its importance please read the following Hadith:

“(1) Narrated AbuZuhayr an-Numayri: AbuMisbah al-Muqra’i said: We used to sit in the company of AbuZuhayr an-Numayri. He was a companion of the Prophet (peace_be_upon_him), and he used to narrate good traditions. Once a man from among us made a supplication. He said: End it with the utterance of Amin, for Amin is like a seal on the book. AbuZuhayr said: I shall tell you about that. We went out with the Apostle of Allah (peace_be_upon_him) one night and came upon a man who made supplication with persistence. The Prophet (peace_be_upon_him) waited to hear him. The Prophet (peace_be_upon_him) said: He will have done something which guarantees (Paradise for him) if he puts a seal to it. One of the people asked: What should he use as a seal? He replied: Amin, for if he ends it with Amin, he will do something which guarantees (Paradise for him). Then the man who questioned the Prophet (peace_be_upon_him) came to the man who was supplicating, and said to him: So-and-so, end it with Amin and receive the good news.”
(Abu-Dawood)

PS: There is a hadith which can be translated some how as: Indeed the prayers (supplications) are between earth and sky. Part of it not goes up, unless you send Durood/salat on your Messenger. (Mudalul Namaz, section Dua with reference to Thirmidhi) - Due to this reason many Scholars recommends to say Salawat/Salat ala Nabi/Salat un Nabi/Darood/Durood before and after making supplication/Dua.

AND ALLAH KNOWS BEST

According to the Hadith I mentioned above, its better to say Ameen even if you are making dua by own.
For sake of ease I am copying pasting again in Orignal Arabic as well:

Arabic:
حَدَّثَنَا الْوَلِيدُ بْنُ عُتْبَةَ الدِّمَشْقِيُّ، وَمَحْمُودُ بْنُ خَالِدٍ، قَالاَ حَدَّثَنَا الْفِرْيَابِيُّ، عَنْ صُبَيْحِ بْنِ مُحْرِزٍ الْحِمْصِيِّ، حَدَّثَنِي أَبُو مُصْبِحٍ الْمَقْرَائِيُّ، قَالَ كُنَّا نَجْلِسُ إِلَى أَبِي زُهَيْرٍ النُّمَيْرِيِّ - وَكَانَ مِنَ الصَّحَابَةِ - فَيَتَحَدَّثُ أَحْسَنَ الْحَدِيثِ فَإِذَا دَعَا الرَّجُلُ مِنَّا بِدُعَاءٍ قَالَ اخْتِمْهُ بِآمِينَ فَإِنَّ آمِينَ مِثْلُ الطَّابَعِ عَلَى الصَّحِيفَةِ ‏.‏ قَالَ أَبُو زُهَيْرٍ أُخْبِرُكُمْ عَنْ ذَلِكَ خَرَجْنَا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم ذَاتَ لَيْلَةٍ فَأَتَيْنَا عَلَى رَجُلٍ قَدْ أَلَحَّ فِي الْمَسْأَلَةِ فَوَقَفَ النَّبِيُّ صلى الله عليه وسلم يَسْتَمِعُ مِنْهُ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ أَوْجَبَ إِنْ خَتَمَ ‏"‏ ‏.‏ فَقَالَ رَجُلٌ مِنَ الْقَوْمِ بِأَىِّ شَىْءٍ يَخْتِمُ قَالَ ‏"‏ بِآمِينَ فَإِنَّهُ إِنْ خَتَمَ بِآمِينَ فَقَدْ أَوْجَبَ ‏"‏ ‏.‏ فَانْصَرَفَ الرَّجُلُ الَّذِي سَأَلَ النَّبِيَّ صلى الله عليه وسلم فَأَتَى الرَّجُلَ فَقَالَ اخْتِمْ يَا فُلاَنُ بِآمِينَ وَأَبْشِرْ ‏.‏ وَهَذَا لَفْظُ مَحْمُودٍ ‏.‏ قَالَ أَبُو دَاوُدَ الْمَقْرَاءُ قَبِيلٌ مِنْ حِمْيَرَ ‏.‏

Complete English Translation:

Narrated AbuZuhayr an-Numayri:

AbuMisbah al-Muqra’i said: We used to sit in the company of AbuZuhayr an-Numayri. He was a companion of the Prophet (), and he used to narrate good traditions. Once a man from among us made a supplication. He said: End it with the utterance of Amin, for Amin is like a seal on the book.

AbuZuhayr said: I shall tell you about that. We went out with the Apostle of Allah () one night and came upon a man who made supplication with persistence. The Prophet () waited to hear him. The Prophet () said: He will have done something which guarantees (Paradise for him) if he puts a seal to it. One of the people asked: What should he use as a seal? He replied: Amin, for if he ends it with Amin, he will do something which guarantees (Paradise for him).

Then the man who questioned the Prophet () came to the man who was supplicating, and said to him: So-and-so, end it with Amin and receive the good news. These are the words of Mahmud.

Abu Dawud said: Al-Muqra’i is a clan of Himyar.

Source:
Sunan Abi Dawud
Book: Prayer (Kitab Al-Salat) - كتاب الصلاة
In-book reference : Book 2, Hadith 549