How to get 5 spectres (with this build)

Hi,

I dont play very often so Im not very familiar with the game. I played like a year ago or so and saw a good video of a build and that made me want to play again.

So, I have this low life spectre summoner. Gear is: shavronne's wrapping, bones of ulr, the vertex, prism guardian and midnight bargain. Specific question is how I get to 5 spectres. Currently, my raise spectre gem gets to level 23. With corruption it can go to 24, right? Bones of Ulr + midnight bargain + unnatural strength = 24.

I think im missing something.

I didnt see te build here but on youtube, and he had the exact same gear and got to 5 spectres. Could somebody briefly explain how I can get one more spectre with that low life build? I know there is other gear that would give more spectres but he had this exact gear and got to 5. But I dont know how to get my raise spectre gem to level 25 for an addditional sepctre.

Thx for any help