At what time can I come to the theater to buy rush tickets for the show on Thursday 20/4 at 8pm? Will ther be good seats? What is the usual discount on the tickets?

There are also apps available. I got reasonably priced tix three days before the show, using the today tix app. Good seats, and very convenient.

I got out tickets in Times Square at lunch time for the evening performance and got good seats

There are three TKTS sales points in NYC (google it), the queues at the one in Times Square are probably the longest. Discounts are typically 50%, though of course not all shows or seating categories will be available.

You can buy tickets at times square and as closer it is to the actual shiw the bigger the discount