I have 15 years Active Army and 3 years with AF reserve with a total of 18 years. Am I supposed to wear 5 or 6 service stripe in the ASU?

SFC (Join to see) from AR 670-1, if I interpret it correctly, you wear 6

a. Enlisted personnel are authorized to wear service stripes as members of the AA, ARNG, and USAR, when they
have served honorably, as indicated below:
(1) In active Federal service as a commissioned officer, warrant officer, or enlisted member of the Army, Navy, Air
Force, Marine Corps, or Coast Guard.
(2) In active Reserve service creditable for retirement for non-regular service, in accordance with 10 USC 1223, as a
commissioned officer, warrant officer, or enlisted member of any reserve component of the Army Forces, including the
Womens’ Auxiliary Corps.
b. One stripe is authorized for each 3 years of honorable active Federal service; active Reserve service creditable for
retired pay for non-regular service; or a combination. There is no limit to the number of stripes worn; however, service
stripes will not cover the chevrons. Service need not have been continuous, and the 10th stripe is authorized after 29 1⁄2
years. Individuals authorized more than 10 service stripes may elect whether or not to wear them.

Each of the service stripes for the Army is for 3 years, that would be 6 stripes for the 18 year total. The Air Force of course doesn't use those stripes but had the Longevity Service ribbon which is four years for the basic ribbon and oak leave clusters for each four year stretch beyond that. Those six service hash marks could be worn on Your Army service Uniform.