Question is in the title.

I know there is a collapseReplicates function in DESEQ2. But I didn't succeed to use it so I did the sum up manually.

In the vignette , 

The purpose of this function is to sum up read counts from technical replicates to create an object with a single column of read counts for each sample

It doesn't say that they divide after sum up...I was wondering if i need to divide the raw counts by the number of technical replicates added because it will have an impact on the size factor ?

Thanks

No, you should not divide by the number of samples. 

And, of course, the size factor will go up, but that's the point of it: After merging several technical replicates, the resulting sample has a bigger size.