each user should execute only 1 iteration and exit & these 20 users should use a ramp-up of 2 users/10 secs.
The best would be to run a constant load policy of 20 users with 1 iteration. Then in the advanced settings of your population you can delay the start of the users by setting a duration.
In your example if you set 100s as delay, NeoLoad will start 2 users every 10 seconds.
For more information you should look at the NeoLoad documentation here