Why when the Jews made the egel, a single bull, they use the words Eilah Elohecha Yisroel which is plural?
The Gemara (Sanhedrin 63a) cites a couple of opinions. (Actually, it's talking about the plural form of the verb העלוך, "those who brought you up," but I would assume that these also fit with the plural אלה. Baal Haturim to 32:4 in fact quotes the second opinion below in explaining this phrase.)
Acherim ("the others," usually R' Meir): they meant thereby to say that they accepted Hashem and (lehavdil) the Calf as co-equal deities (shituf).
R' Shimon bar Yochai: they wanted to worship many other deities besides the Calf. (He considers this less objectionable than shituf.) This opinion actually has halachic implications too: once the Jews demonstrated that idolatry was acceptable to them, then the Canaanites living in the Land of Israel could be considered their agents, and their worship of things such as asheirah-trees would indeed make them forbidden for use, even though they were already part of the Jews' patrimony (Avodah Zarah 53b).
The Ramban explains the function of the gold calf, that it was not meant to be an object of deification. The people were saying that the spiritual powers of this calf-like image brought them through the wilderness. This means the spiritual power associated with the image-the attribute of strict justice-and not the image itself. My suggestion, based on this, is that Eileh is referring to those spiritual powers, ie plural
Actually, I've now noticed that earlier in in the parsha in posuk alef, it says Asay lonu elohim asher yeilchu... and Rashi says Elohus harbay ivu lohem - they desired to have many
The Yerushalmi, Sanhedrin 10:2 explain this in the name of R. Shimon bar Yochai, they made thirteen calves, one for each one of the shevatim and one which was common to all of them:
תני ר"ש בן יוחי שלשה עשר עגלים עשו ישראל ואחד דימוסיא לכולן ומה טעמא ויאמרו אלה אלהיך ישראל הרי לשנים עשר שבטים זה אלהיך הרי דימוסיא אחת לכולן.