For me I bought loads of outfits...never used them 😂
I found footed sleepsuits and short sleeved vest were easiest for layering and managing temperature and the zipped sleepsuits easiest for all the nappy changes.
Your baby will be entering summer so footless would be useful if it's really hot.
I wouldn't bother with pramsuit, if he's born in April you won't get much use. With my baby i just wrapped him in different thickness blankets to go out (obviously layered vest, sleepsuit and thick cardigans depending on weather) and our buggy had a fleece warm cosy. You would have to take it off everytime you put baby in the car, so depending on your lifestyle it is more of a hindrance.
I did get a baby monitor with a room temperature sensor and used the online guidelines posters for temperature and layers.
For first few weeks I found swaddle blankets a lifesaver to settle him, then sleepbags as soon as he started moving more at night.
I second NEXT. I got an account with them, they have so much variety and different brands too. Even organic 100% cotton (my eldest had awful eczma and as she's got older she's allergic to everything, put her in certain fabric and she's gets rashes)
As for how many. Depends on your baby. My boy had colic and would be sick constantly so would go through so many sleepsuits in a day. At NEXT they do multipacks so I would grab couple of those for around the house and then the cute ones for going out.