How many significant figures does have 1.1+2.345+5.05?

The number of significant figures the answer to a sum will have (unless you round it), will be equal to the maximum number of significant figures in the question, unless there are multiple with the same. To double check you can just do the sum, in this case the answer is 8.495.

The number of significant figures is the number of figures in a number starting from the first non-zero number. I. e. 00000000102 still only has 3sf.

So in this case, our number has 4 significant figures.