what is the relationship between increasing the number of cores and scalability? Whenever I increase the number of cores the speedup will increase. with this increasing can I consider the system as scalable? 

I mean based on the speedup obtained for a different number of cores e.g. 10,20,30, is the system scalable with the increase of cores?
If the problem size (input data set and/or computation) is fixed , then NO. Amdahl's law says that. Speed up will be bounded by serial portion of the program.

If the problem size (input data set and/or computation) increases with the increase in the number of cores, then YES, Gustafson's law says that.