As a temporary counter for sentences, I found a site that will do just that.
https://wordcounter.net/Yeah, it's called "word" counter because it counts words. But, if you look off to the side and notice all of the numbers, you'll see that it will count the amount of sentences you have done, too. There is an option to change the settings to count sentences only to make it easier for you.
How to do that:
-Go down to where it says "Options"
-Next, switch off, or on, all the things you want to be included and excluded.
And there you go! Oh, you can also set a goal that you want to reach. Right now, I put up a goal for 600 sentences to be done. So every time I type up more sentences, it keeps track and tells me how much closer I am to reaching that goal.
Hope you find this helpful!