Look at the space you would want for your 45 fish when they are adults. Unless you can easily move some of them out, that is what you will have in just a few months. For guppies, you could probably get away with a 29 gallon that is well maintained. If you don't want to do frequent large water changes, you might want to go to a larger tank.
If you are running that 2 gallon toy without a filter, do daily 75% or larger water changes.