These ideas may get you moving forward.
1)
In Simulink (unlike MATLAB) signals have fixed maximum lengths and most commonly the length is fixed. You need to design what you want to happen when the fixed limit is exceeded. Keep only the first N values? Keep only the last N values?
2)
For the implementation inside your triggered subsystem,
Use unit delays to hold on to two persistent values.
First, Integer valued scalar representing index_counter of which trigger this is. 0th, 1st, 2nd, ...
Second, vector of length N that holds up to N trigger values.
Feed the unit delay outputs back into themselves but with a little bit of logic on the feedback loop.
For example, feedback loop on index_counter should be increment the value whenever a new value is stored. And if using a circular buffer scheme, some logic to wrap the value back to zero when it exceeds N-1.
Feedback loop on vector signal would use indexing to change just one value in the vector
