Multi values for Last-N

Dear All,
I can see that we can set dynamically the value of Last_N based on the number of participants. The syntax in the config.js is the following:
lastNLimits: {
5: 20,
30: 15,
50: 10,
70: 5,
90: 2

Is there an easy way to set this value using a param passed via URL link ?
Other question, I would like to understand the selection of the Last-N video streams. If I set Last-N=15 and the room has actually 30, then only the “first 15 participants” are sent to every member of the room, aren’t, they ?. Are the first participants determined by the time order to join the room ? And if one participant among these first participants loses connection, rejoining the room afterward, he is no longer visible because he becomes now the last one who enters the room ?
Many thank for your explication

Last-N means “last talked N” and the list is updated when a new one speaks

Thanks @emrah,.
I see that this parameter is set for conference object. Does it mean that its value is common for all participants? In other words, we can not set so that in the same room, some participants have greater Last-N than those of others (they have more powerful PC for instance…), can we ?