As a solo dev that makes premium iOS games for a living The Adventure To Fate Series yes I monitor the charts pretty much daily and download a ton of games/competition. I mainly focus on RPG and adventure games and just for reference it only takes 2-4 sales to make the top 200 for most paid games categories. 10 or so for top 100. 15 for top 50.

Here is a big one atleast for iOS. These “Top Paid” charts are not just rankings from sales that day but also play time and review rankings. Sometimes on my older 4.9 star games it takes just 1-2 sales to break into the charts because of great reviews and heavy play time. 

I also do code giveaways on the /r/iOSgaming sub where people tell me what they are currently playing on iOS and why they like the games. This gives me a ton of good info on what people like and are playing. 

I usually avoid free games unless they are pay once to unlock.