population is 100000, they in 250 1AB, 1C category of schools in 3 district. I want select the sample considering school category, district, and gender equality . how many unite should I select? For survey study based on these three objectives (MPhil/PhD)

To identify the present situation of selecting the subject scheme of collegiate-stage students for their advanced-level examination in Sri Lanka

To Explore support given by the schools to collegiate-stage students to select their subject scheme for advanced level examination in Sri Lanka

To reveal the present neediness frequency for a subject selection model for Collegiate-stage students to select a subject scheme for their advanced-level examination in Sri Lanka