I think what’s possibly happening is that by artifically limiting your reviews so drastically, you’ve created a bottleneck of due/overdue cards. That’s going to keep you from being able to get anything but those cards, because Anki knows to prioritize them. [If you want to see what you have stacked up, search prop:due<=0 in the Browse window, or go to Stats, and turn on “Backlog” in the Future Due graph.]
If you have a backlog, those 12 cards that you’re adding each day are somewhat pointless. You’re not going to see them again until after your backlog cards, when they are long overdue and more likely to lapse.
As for your overall plan –
Reviewing ahead should be used sparingly, and (as you saw above) review limits can be troublemakers because they hide the true number of cards you need to be studying. You should raise your review limit. The Options screen is telling you this for a reason:
If you want to smooth out your peaks and valleys, a better way is to turn on FSRS, and then use the “Load Balancing” feature of the Helper add-on.
After the optimal interval is calculated, it is adjusted by a random amount to make the distribution of reviews over time more uniform.
That one deck in particular is pretty new and there is no backlog, I just learnt something like 20-30 cards, so there should be no bottleneck whatsoever. I just think that since I’ve learnt the cards, it does not add them to reviewing and there aren’t enough learning cards to reach the 20 limit.
Also even if I reset to defaults setting, I still have few reviews
Oh good! If you haven’t been limiting it very long, there hasn’t been time for a backlog to form. In your Stats page (Statistics - Anki Manual), you can look at your Future Due graph to see what the forecast is for how many cards you’ll be reviewing each day. This only counts currently active cards, not any New cards you’ll be adding. (If you want to simulate New cards, try the Anki Simulator add-on, https://ankiweb.net/shared/info/817108664)
You have absolute control over how many New cards you introduce each day – but you cannot really control when they come due. Your part is to grade your answers honestly, and then Anki (using either of the scheduling algorithms) takes over, attempting to schedule them when you need to see them to keep your memory fresh.
You should expect if you continue adding 12 New per day, you will stabilize at around 120 reviews per day (assuming default settings). If your priority instead is to have around 20 reviews per day, then you shouldn’t add more than 2 New per day.