1. how many amperes of lead acid battery i'm gonna use if my solar panel is 50w?
This question doesn't really make sense.
You might be asking "what charge rate in amps could I expect with a 50W solar panel?" Even tha is incomplete because we don't know the system voltage. Let's assume it's 12V. In this case your maximum charge current is going to be around 4A.
You might be asking what current can you draw from the battery. This is determined by the battery itself, and might be 10A or 1000A (or anywhere in between) depending on the battery.
There are probably other things you might be meaning too.
2. what is the use of charge controller? can i directly connect the solar panel>> lead acid battery>>12v devices?
A charge controller performs a number of functions:
1) It prevents your batteries from being over-charged.
2) It manages the charging process to try to maintain your batteries in good condition.
3) It prevents the battery from discharging through the solar panels at night.
4) It may employ techniques to obtain the maximum power from the solar panels (MPPT)
5) It will often disconnect the load to prevent the batteries from being excessively discharged.
If you can do without all of these things then you have no need for a charge controller.
3. what's the meaning of specification written in solar panel (Maximum system voltage: 800V)?
In grid connect systems many panels are connected in series to give a high DC voltage. This rating limits the maximum voltage of such an array. It is essentially an issue of insulation.
