A 10,000mAh battery will give your phone around 6,000mAh of extra battery charge. Apple doesn't publish its battery capacity, but teardowns have shown that the iPhone 15 has a battery with around 3,350mAh of battery life. That means a 10,000mAh battery can charge that phone twice ... almost. You may not get to 100% the second charge, but you'll be in the 90% range.
Any power bank that is 5,000mAh or smaller will probably not guarantee 100% charge on an iPhone 15 or larger phone. If you have a big battery phone like the Galaxy S24 Ultra, which packs 5,000mAh inside, you'll need a power bank closer to 8,000mAh or more if you want a full recharge.