2. Describe the main types of instructional activities that could be used to support the students learning of basic number concepts and addition and subtraction of sums less than 20 and why would they be included.
See the attached file.
Mathematical concepts for first grades do include the concepts that your posting mentions - but, to back up what the posting says, I would look up the math curriculum standards for any state that you might choose and see what they say - this gives you a scholarly reference to cite for any paper you might have to write on the topic, and also provides material for comparison.
For instance, in this .pdf document of Georgia's math standards for K-grade 2, the performance objectives for first grade includes that students know numbers up to 100, not to 20. This document is attached as part of your solution so you can see it and compare.
This document will provide you with the answers to question 1.
UPDATE: This .pdf ...
The solution discusses concrete, observable, quantifiable curriculum learning objectives for first grade math, including a Web reference to Georgia's curriculum standards for first grade in Math.