The currently accepted answer includes a table that states the United States has ratified 5 of 14 (not 18) international human rights treaties. This answer is intended to provide some context on the 14 versus 18 aspect of the question.
The Office of the United Nations High Commissioner for Human Rights (OHCHR) considers there to be 9 core international human rights instruments:
- Convention on the Elimination of All Forms of Racial Discrimination (ICERD, 1965)
- International Covenant on Civil and Political Rights (ICCPR, 1966)
- International Covenant on Economic, Social, and Cultural Rights (ICESCR, 1966)
- Convention on the Elimination of All Forms of Discrimination Against Women (CEDAW, 1979)
- Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment (CAT, 1984)
- Convention on the Rights of the Child (CRC, 1989)
- International Convention on the Protection of the Rights of All Migrant Workers and Members of Their Families (ICMW, 1990)
- International Convention for the Protection of All Persons from Enforced Disappearance (CPED, 2006)
- Convention on the Rights of Persons with Disabilities (CRPD, 2006)
(The United States has signed 7 and ratified 3 of these 9 core treaties.)
In addition, there are 9 optional protocols associated with these instruments:
- Optional Protocol to the International Covenant on Civil and Political Rights (ICCPR-OP1, 1966)
- Second Optional Protocol to the International Covenant on Civil and Political Rights, aiming at the abolition of the death penalty (ICCPR-OP2, 1989)
- Optional Protocol to the Convention on the Elimination of Discrimination against Women (OP-CEDAW, 1999)
- Optional Protocol to the Convention on the Rights of the Child on the involvement of children in armed conflict (OP-CRC-AC, 2000)
- Optional Protocol to the Convention on the Rights of the Child on the sale of children, child prostitution and child pornography (OP-CRC-SC, 2000)
- Optional Protocol to the Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment (OP-CAT, 2002)
- Optional Protocol to the Convention on the Rights of Persons with Disabilities (OP-CRPD, 2006)
- Optional Protocol to the Covenant on Economic, Social and Cultural Rights (ICESCR-OP, 2008)
- Optional Protocol to the Convention on the Rights of the Child on a communications procedure (OP-CRC-IC, 2014)
(The United States has signed 2 and ratified 2 of these optional protocols.)
These 9+9=18 instruments are presumably "the 18 international human rights treaties" referenced in the question.
The 14 treaties mentioned elsewhere include the 9 core treaties, 4 of the optional protocols, and 1 more not included above:
- Interstate communication procedure under the International Convention for the Protection of All Persons from Enforced Disappearance (CED Art.32, 2010)
Some other lists online also include instruments such as HURIDOCS, OMCT, ICSPCA, CRSR, Convention on the Reduction of Statelessness, CPPCG, and ILO.