Why SLC sentinel-1 product has 18 intensity and Amplitude

Why SLC sentinel-1 product has 18 intensity and Amplitude which is useable for soil moisture estimation

you can use thea search function to see if your questions were already answered elsewhere:

For example: Difference between amplitude and intensity

Soil moisture: Soil moisture identification -Sentinel
Also: Soil moisture mapping using Sentinel-1-Data and SNAP?

ya but i want to know which image to use from 18 SLC images

they all belong to the same product: Each IW product consists of 3 swaths (IW1, IW2 and IW3) and each swath consists of several (mostly 9) bursts: It is explained here:
https://sentinel.esa.int/web/sentinel/user-guides/sentinel-1-sar/acquisition-modes/interferometric-wide-swath

In case you have dual-polarized data (VV+VH) you have the same area imaged twice per product (both vertically and horizontally receiving).

do i have to use both amplitiude and intensity during processing for soil moisture

no, they are the same measurement. But it would be good to calibrate the data before analyzing it. Please have a look at this topic: Radiometric & Geometric Correction Workflow

Retrieval of soil moisture with SAR is surely challenging. Based on your previous questions I can suggest to read some of the basics on radar processing (or have a look at the tutorials of ESA or ASF) before getting into soil moisture retrieval so you have a better understanding about what you are doing. There is no standardized solution on how to derive soil moisture and you surely need data collected in the field to calibrate your prediciton.

thank you for your concern