How often can I contribute to an traditional IRA CD account before its maturity date? If I open an 18 month IRA CD, can I add to this account before 18 month mature?

You can contribute to your 2018 IRA before April 15. Whether that contribution can be in the form of an addition to your CD is up to the financial institution that holds the CD. 

For tax purposes you only have ONE Traditional IRA which can be split between as many different accounts as you want, but for tax purposes they are all added together.    The IRA contribution  limit applies to all IRA accounts.   Some accounts can hold both CD's and non-CD money - depending on the financial institution.  You can always open a new IRA CD account or just an IRA account  for new contributions.