Batch Apex - Call Out

What is the Call Out limit from Batch Apex ?

As per latest the upper limit is 100

Any workaround to process more than 100 call outs ?

Database.executeBatch(new BatchClass(),100) 

Pass the scope parameter as 100,Which can split the batch execution into multiple batches

How to define the scope w.r.t the callouts?

In execute method you have 2 call outs ,Your batch size should be 100/2 = 50

execute method has N callouts your batch size should be 100/N

Which interface need to use to implement call out from Batch Apex?

Database.AllowCallouts

What happens if we don't implement this interface and try to do call out ?

First error: Too many callouts: 1

Which type of call outs are allowed ?

HTTP request as well as methods defined with  web service key word .

Call Outs are allowed from Start /Finish methods?

Yes,Its possible 

REST API Call from Batch Apex :