i want to know about how many user can be log in fronted magento ,and how many user limit at a time in magento website.
As many as you have resources for:
Where you store your sessions matters:
So there is no technical limit, only a financial one.
It almost belong to the storage engine you use to store session. If you have the huge user session number login then I highly recommend you use Redis as storage engine for magento. Redis will know how to optimise the memory and get the best performance. You just need to provide the hardward good enough.
You can check this for more detail how to use Redis for magento https://github.com/colinmollenhour/Cm_Cache_Backend_Redis
Other than that, when you use Redis as storage engine for session, you will make it faster file system and database storage and release the load for file and database.