Zeh lo oveid kacha.
When a seat is taken from XN it goes away from X as well.
+1
Warning: major digression ahead, with plenty of technical details. Hard hats required... as well as patience. Higher buckets overlap lower buckets - like we say,
"b'chlal ma'asayim maneh". For example, if you look at revenue biz tickets, and you see J8 C8 D7 Z5 P2, that does
not mean there are 30 seats (8 + 8 + 7 + 5 + 2) available; it means there are 8, as follows:
2 seats available in whatever fare class you want,
5 that are available in J C D or Z (but not P),
7 that are available in J C or D (but not Z or P),
and 8 that are available in J or C.
If you take two seats in P, then in theory the availability will drop to J6 C6 D5 Z3 P0 - each bucket loses 2 seats, not just the bucket you took from.
(I say "in theory", because in practice, airlines can do whatever the heck they want... and they often do. If the computer wants, it can suddenly make all of them only available in J, resulting in J6 C0 D0 Z0 P0. Or if they want to sell everything fast, they can put everything into P, resulting in J6 C6 D6 Z6 P6. But I'm digressing, which is why this is in small print. ) Likewise, if you take 5 in D, that won't result in only D going to zero; Z and P will (generally) go to zero as well with it.
Complicated enough?