There are 53 numbered pins and A0 through A15 for analog. Can I use all of these pins at the same time?
Well, typically, it'll take 100us to read each analogue input, and because they share a single A/D converter, they'll have to be read sequentially.
You better be careful about total current use. Actually look it up even.
Using all pins at exactly... the same time is a bit difficult, but you can use all pins for a project. With a bit of luck you can program them to operate fast enough for your project.
Anyway, including pin 0 there are 54 digital-only pins and A0-A15 can be used to read analog values as well. If the controller operates within its parameters (check datasheet !) you can... use all 70 pins.
2 pins, 0 and 1, are already in use to communicate with your PC, but if you really need to, you could use 'm for other purposes as well.
Do you have a project in mind ?
Just 5 mA on 50 pins is 250 mA. An Uno isn't supposed to exceed 200 mA, how about the Mega?
What limit should you be choosing resistors to fit? But you look.