We started (and ended as we are retired and don't want to spend more time working!) with 6 units. I wish we had gone larger -- here are my reasons:
1) Common area maintenance will essentially remain the same between 6 and 24 units. especially if you can avoid a building with interior corridors. Your landscape footprint will not quadruple between 6 and 24 units.
2) You will be remodeling each unit on a one or two at a time basis anyway -- it will take longer because you have so many more units. No experience? You WILL be experienced after you complete your first unit remodel. You will get more efficient with each remodel, you will be able to correctly know what equipment and supplies will be required for each remodel. You will be able to take advantage of economies of scale and bid desk discounts.
3) Easier for you to live on site. Live in one bedroom and use the apartment living room to store supplies and equipment. You will be able to quickly evaluate which tenants you want to keep versus not renew, since most tenant issues arise after business hours.
4) Youth and energy is on your side. If you totally crash and burn, you will have many years to recover financially. Youth is the time to take risks. (not so much for us old farts!)
This IS a huge project, but the payoff is so much greater. If you are single, sacrifice your social life for a year or two for financial independence. If you are married, make sure your spouse is 100% on board with this project, since you will have many evenings of painting, laying floors and remodeling ahead of you.