I will accept as many players as sign up, and game size will depend on how many we get. I may adjust the settings if I feel it necessary.
Minimum 5+ per game? up to 12 players per game or? Will you try to go around 5-7 players per games?
Can I raise to 15 whenever I want? I am not used to poker and may not understand everything correctly.
If someone put 3, then everyone else has to put 3 or "fold" (= give up but keep remaining chip to another game? ) ?
For the first round i'd like to start with at least 7 player games, but it depends how many sign up. As players are eliminated the games games will get smaller, and when two groups are sufficiently small they will be collapsed together. This will continue until there is only one active, the 'final table'. Clearly, this could become just 2 players at some point, but there will only be 2 player games if they are the last 2 players in the tourney. In short, yes I will aim to have at least 5 players in the majority of games.
You can only raise in the time between the end of the turn of the person before you, and before you place your final deployment. You cannot make any actions outside of that window, and an attempt to do so at an incorrect time will not be counted. If no action is made during your action time frame, then the default action is applied to you, which is to call - That is, to put in exactly the number required to stay in the pot, regardless of what that number is.
Providing you are acting within your action window then yes, you can raise to 15 'whenever you want'. There is no maximum bid.
And to your final question, yes that is correct.
Please do ask me to clarify if anything is still unclear.