Comparing overdubs for the song against others in Carpenters canon, I find the overdubs--in this One instance--lacking.
(Am I the only one ?).
Perhaps. I've just done a second-by-second comparison of the amount of time given to overdubs in both "I Believe You" and one of the quintessential Carpenters records, "(They Long To Be) Close To You". That was the song that propelled them to number one and basically defined their formula.
In "I Believe You", a 3:54 song, I find 1:48 worth of overdubs, for a total of 46% of the record's time.
In "Close To You", a 3:38 song, I find 1:27 worth of overdubs, for a total of 40% of the record's time.
In fact, much of "Close To You's" overdub times comes from a doubling of Karen's voice rather than the almost total stacking found in "I Believe You".
I'm not suggesting that there aren't more songs with more overdub time, but I'd be willing to bet that there are quite a few with less, particularly in the later years.
Harry