Are there planes available at the drop of a hat,
Rarely.
When an airline has an aircraft out of action for some reason and its likely to be more than 24hours they will often lease in an aircraft for the duration.
I watched Ryanair in action one day.
First inbound went tech.
They took the next arrival and used that as the outbound leg.
They kept doing it until the executive jet arrived with a technician to fix the first aircraft.
Consequently the 3 outbound flights were all at least 1 hour late.
But no EU261 compo due.
Clever.
Bear in mind that there were no based aircraft and all were 800 series,
How come they knew about the lack of seats in relation to the poster who got allocated a non existent seat 00.
Ryanair system tracks what plane is scheduled to cover what leg.
Typically all based aircraft will be the same for small bases (Shannon/Cork).
This simplifies logistics.
Larger bases (Dublin) may have a mix of types.
So when there is a problem the next "available " replacement may be a different model.
Paying compo to 6/9 passengers is better from a corporate perspective than all passengers.