I've found that bowling 5 stock balls per over has the batsman to comfortable.
I get better results with more variation but not every ball.
Another method to improve your bowling is to go into replay mode during the match and watch the AI's bowling. This can give you an idea on how to bowl an uncomfortable and dangerous over for a batsman. The same applies for watching real cricket but this makes it easier to translate into something you can do in the game (Slow bouncers can be tricky in real life but useless in CR19).
Doesn't matter if its your teammate or the opposition, just try and find a bowler who uses the same style as you (bowl style, swing, seam, leg spin, etc.) and watch their lines, lengths and delivery. After carefully watching probably 100+ overs in replay mode, I actually saw an AI bowling cross-seams yesterday. They might have bowled them earlier but I never observed it as a batsman/fielder. You will get to learn the different delivery types by looking at the seam placement, and also the right lines and lengths to use them on.
Anyway, even if your struggling on a particular pitch, watching how the AI plays to the field set up and pitch can help you improve a lot. As
@Nige said, you'll even see the AI doesn't really change delivery type, its more about length and line but even then don't overdo it (except in T20). The same plan won't work on every team on every pitch but if you have an idea on how to throw some effective deliveries, you'll notice significant improvement.
I'm using a build that gives me a really high "stock ball" as I see my standard up and down seam as my "fast ball". Bowling at 160km/h just outside off or bouncing over the top of off is a really hard ball to hit, even for the AI. This sets up some subtle variations with the use of a few leg cutters, an occasional off cutter and the intermittently sneaky cross seam at full length. Short balls/bouncers are also lethal when combined with intense speed.
I noticed that in Cricket 17 when aiming I presumed it was like EA cricket games where you moved the direction indicator totally to the left or the right but doing that in these titles means the ball barely pitches on the cut strip, regardless of the type of delivery.... You only need to move the direction stick slightly to the left or right to change line. And the more subtle you are with it, particularly when using swing/seam movement, the more results from false shots and swinging bats you get.
Line direction to a RHB over the wicket with 50 line sensitivity:
Point your line at St
andard: Outside off stump.
Point your line at Stan
dard: Over/outside leg stump, sometimes mid stump.
Point your line at Sta
ndard (no change to line): Tends to bounce over/just outside offstump but can do something off the pitch and end up leg side.
Point your line at S
tanda
rd: Ball ends up closer to the tram lines, good for forcing bad drives.
Adjust your lengths and speed for variation.