I created a hotspot on my laptop with My WIFI Router software.I'm wondering how many devices can be connected?
You'd have to ask the folks who wrote My WiFi router about that. Limitations can vary. Each connection probably takes up a small amount of RAM and processing power, on top of bandwidth. There might be a set upper limit, or the software might just let you keep adding devices until something breaks.
Will too many devices would cause slow connection?
If they're all trying to access data at once, then yes. Each device has to share the same speed data connection, so if multiple devices use data at the same time, all of the devices will see slower data.