I have two spare laptop hard drives and I want to use them with my raspberry Pi4. I wonder if a 5v2a power supply enough to make them up and running. If a 5v2a power supply wasn't enough, which kind of power supply should I purchase?
A 'portable' 2.5" USB HDD can take bursts of considerably more than 500mA at the time of spin-up, up to 1000mA or more, and also when seeking or carry out extended reads/writes. Also, the total current available from all of the USB ports is 1200mA, so using 2 USB drives at once is VERY unlikely to be successful, even with the official 5.1v 3.0A (3000mA) power supply, let alone a 5v 'charger' or PSU with a lesser current rating. I have tried 2 USB powered drives at once (one Toshiba Canvio 1TB and one WD Elements 2TB) with an RPi 4 and was rewarded with loud clicking noises and very erratic operation. To connect two drives, at least one must be independently powered, e.g. by a powered USB hub.
If you are talking about 'laptop' hard drives (bare 2.5" SATA drives) you will have to house them in enclosures, or use USB-to-SATA adapters, which may introduce some further complications with the Pi 4, covered by a sticky at the Raspberry Pi official forum. That forum is an invaluable resource which should probably be the first place an RPi user goes to in times of trouble.