I had a cucumber Saison that managed a very nice cucumber flavor--wasn't sure I'd like it at first, but I was pleasantly surprised. As others have said, I think you'll need quite a lot to get significant flavor, but you can also start with a more modest amount, taste after a few days, then add more if you can't taste or smell it enough.
Just out of curiosity, are you adding to a keg, or secondary before bottling?
Totally guessing, but I'd probably start with at least 6-8 finely sliced cucumbers in a 5 gallon batch. I'd put them in a couple weeks before you plan to bottle/serve, and plan to allow for delayed bottling/serving if you taste and feel like you need more.