In each theme under the the customise > theme settings you should see options for
Homepage Project Count
Homepage Blog Post Count
Portfolios Page Size
Project Lists Page Size
Blog Page Size
Homepage Project Count
This will dictate the maximum number of projects that show on the homepage
Note: if you use a homepage portfolio to organise homepage projects and it only has 3 projects in it and the count is set 20 it can still only show 3.
Homepage Blog Post Count
If your theme supports it normally blog posts are listed after the homepage projects and this number will dictate how many show there
Portfolios Page Size
This will dictate the number of portfolios that get listed on your show all portfolios page /portfolios
Project Lists Page Size
This will dictate the number of projects that show in your list all projects page /portfolio and the number of project that show in a portfolio
Blog Page Size
This will dictate the number of projects on your /blog page
Alongside those options depending on you might have additional settings for counts.
Homepage Featured Count
This will dictate the number of projects that show in the featured area of the homepage. But the Homepage Project Count has to be considered so for example if you want 4 featured items and 8 below your Homepage Project Count has to be 12 (4+8) as thats how many projects are returned to the page and 4 are designated to the feature area leaving 8 to go below.
Note: When using a Cover layout for homepage it is similar, that 1 is designated to cover area and rest are below, for example if you want 1 cover and 10 projects below the Homepage Project Count has to be 11 (10+1)