Different number of lessons
When i look at my friend's ipad, there are less lessons for each skill than mine. For example, his adverbs has 10 lessons while mine has 12, why is this?
Right you are, that's what I get for not reading carefully. It still sounds like one of Duolingo's A/B tests, but as far as I know, that num_lexemes_per_lesson test only affects the web lessons, not lessons on the app.
It might be worthwhile to compare your ab_options data with that of your friend to see which test if any is affecting you. The easiest way to do this is to open your browser's Developer's console and enter "duo.user.attributes.ab_options". That should return to you a list of all the current tests and their values for your account. Most, if not all, of the tests that affect the iPad app have names prefixed with "ios_". Comparing your values to the values your friend sees for those tests would be a good starting point.
It depends on the browser you're using. I know it's Ctrl+Shift+J for Chrome and Ctrl+Shift+K for Firefox. If you're using another browser, you'll have to explore around in the menus or google it.
Also, on a slight tangent, when you want to respond directly to someone in the forums, you should do so by clicking the "Reply" link at the bottom of their post. That'll do a couple things, it'll make sure that they see your reply since they'll get a notification message, and display your reply indented beneath theirs, making it easier for others to follow when they come by in the future.
The operating system shouldn't make a difference, it's all about the browser. I don't know a lot about Mac's default browser, Safari. It looks like the shortcut to the Console is Option+Cmd+J based on this result from a quick googlin', but I can't help you much beyond that having never used it myself.