What kinds of religious jewelry, if any, are allowed to be worn at Basic Training?

You are allowed 1 I believe.
(a) Soldiers may wear religious apparel, articles, or jewelry with the uniform, to
include the physical fitness uniform, if they are neat, conservative, and discreet.
“Neatconservative,anddiscreet”isdefinedasmeetingtheuniformcriteriaofthis
regulation. In other words, when religious jewelry is worn, the uniform must meet
the same standards of wear as if the religious jewelry were not worn. For
example, a religious item worn on a chain may not be visible when worn with the
utility, service, dress, or mess uniforms. When worn with the physical fitness
uniform, the item should be no more visible than identification (ID) tags would be
in the same uniform. The width of chains worn with religious items should be
approximately the same size as the width of the ID tag chain.
(b) Soldiers may not wear these items when doing so would interfere with the
performance of their duties or present a safety concern. Soldiers may not be
prohibited, however, from wearing religious apparel, articles, or jewelry meeting
the criteria of this regulation simply because they are religious in nature, if wear is
permitted of similar items of a nonreligious nature. A specific example would be
wearing a ring with a religious symbol. If the ring meets the uniform standards for
jewelry and is not worn in a work area where rings are prohibited because of
safety concerns, then wear is allowed and may not be prohibited simply because
the ring bears a religious symbol.
(c) During a worship service, rite, or ritual, soldiers may wear visible or apparent
religious articles, symbols, jewelry, and apparel that do not meet normal uniform
standards. Commanders, however, may place reasonable limits on the wear of
non-subdued items of religious apparel during worship services, rites, or rituals
conducted in the field for operational or safety reasons. When soldiers in uniform
wear visible religious articles on such occasions, they must ensure that these
articles are not permanently affixed or appended to any prescribed article of the
uniform.

I wore 2 gold chains throughout my time.

I can't see why they would issue you one. I doubt if they issue crosses to Christians, Stars of David to Jews, etc