Let's see ,i don't know if your are agree guys .but a series with 12 to 15 episodes like:
onegai teacher,elfien lied ,rozen maiden ,kimiga nozomu eien .mahoromatic,dears,happy lesson ,hanukyo maids ,dna2,must have one op/ending . sereis with 24 to 26 like:
chrono crusade,chobits ,full metal panic ,mai-hime,scrapped princess,dnangel,evangelion ,star ocean ex ,must have 2 op/endings ,except chrono,only change the opening but the song is the same ,and the rest of the series like ,shaman king,yugioh ,gto,gragon ball gt,naruto ,inuyasha ,flame of recca ,and more must have more endings and openigs like full metal alchemist ,4 op/2 or 3 endintgs i don't remember well i did not put much attention ,sorry ,any way ,some animes doesn't have many .i want to know your opinion please.what do you think?

I think I'd go for the 24-26 episodes. But yah know, if the story is really nice I'd want it to go longer. Other animes which have lesser episodes, was just bec. you can't expect them to run the story long when they have nothing else to add to it.

I don't know, but I suppose that they need to have the sufficient ones to delight the ears of the fans n_-

Ayashi no Ceres is definitely a must have! .. it's one of my favorite openings ever ^___^- .. One of the Slayers openings is nice too.. I forgot which one though =X .. ^^;; ..

[edit]
Wait.. do you mean.. song-wise or animation wise?

Hum.. i think just one, and good, open and ending are cool! ^^

Sorry I dont know

More likely if the toy company sponsor anime series and that become a hit then different opening and ending will be added. Look at Gundam for example.

I think the more the better because i love anime music.. jpop etc.. and, everytime naruto or for example Tokyo Underground had 2 op and 2 ed and it was just 26 eps.. i like when they change.. it's a form of saying the anime is evolving .. ^^, or well in my point of view.. (y heh =^.^= holaaa alex-kun.. me gustaron tus threads.. muy buenos ^^.. cuidate besos, bai)

Well I don't know which is the number exact of endings and openings, I like so much listen the songs , so if are many endings and openings is better.

I think series that are around 26 episodes should have 2 openings and 2 endings while a series that is around 50 should have about 4 endings and openings. I series any longer (which I really do not like to watch when it get to long) should have a new opening and ending for every new season it has. But that is just what I think though

anime series of 9-15 episode should have 1 opening/ending
and 24-26 episodes should have min. 2 openings/endings
and series off 50 episodes should have at least 3 openings and endings

I love anime OSTs, but one opening theme and maybe two ending themes are just fine (as it was in Chobits) ...

Depends. Usually, one opening is good enough, with three to four endings.

Just like you said, and they should change the song (music junky)
I would have liked Eva to have another opening

i think that one opening is good
but multiple openings are nice because it adds sort of a new feel
but for me i'd just pick the songs that sound good to my ears ^_^

For me it doesn´t matter how many openings an anime have got! I want to watch the movie and most of the time I skip the opening.

Around 12 ep, one should do. Around 25, 2. 50? 4.

I think the opening and the ending could be change by anime season or when the anime theme changes.

I think whenever the first ones get old. Or the series is so long, the opening/ending can change with the upcoming events in the series.

just stick to 1 good ol'faithfull OP and 1 ED

I don't really have a preference as to how many different openings or endings an anime has- 1/2 is nice 3/4 is cool- as long as they are really good- all that matters is the quality not the quantity...i'd rather have 1 opening and 1 closing i love and enjoy to see over and over again, than 4 different op.s/end.s i don't like

Except for a couple of series, I would rather not have any at all. Except maybe if they did a different opening / ending every episode. Ex : The last few Bleach show different "Gods", that was a little entertaining.

I also think it depends on the number of eps.. 24/26 should definitely have 2ops & 2ends.. I mean, i just saw Fruits Basket and was really dissappointed with the OST.. i mean, only 1op and 1 ending, and all the background music was slower or faster versions of those 2 main songs! That's really a lack of creativity! Although the songs where nice, it was kinda boring!
60/+ episode series should have AT LEAST 4ops/ends.. I like it when there a LOT of music to enjoy! Music is the difference between a good anime (which can come from an excellent manga) and an EXCELLENT anime! Without it, the manga and the anime would be the same thing!

I don't mind any number of OP/ED, as long as they are great. I admit I won't be able to stand one OP/ED for each episode though.