If a game's abandoned they give out vouchers as you leave. Remember it's not just season ticket holders but also people who paid at the gate.
You always get extra tickets in the book- better safe than sorry. They can be used as vouchers, for example to give season ticket holders priority if we drew a big club in the cup and there was a scramble for tickets.
Mind you that's usually more common at bigger clubs - at ICT there seems to be no bother ringing up the stadium and just telling them you're in a priority group.
18 home matches surely???? You don't usually get into cup games with your season book, but sometimes a club will include an early season tie or two in order to sell more season books - say the CIS or Intertoto/Europa League etc.