Can I wire my guest bathroom on one circuit?
Yes, in the US under the NEC you can wire a single bathroom on a 20A circuit, provided that circuit feeds ONLY loads within that one bathroom. This circuit cannot be shared with other rooms or other bathrooms.
This is not to say this is best practice, but it can be done.

210.11 Branch Circuits Required
(C) Dwelling Units.
(3) Bathroom Branch Circuits.
In addition to the number of branch circuits required by other parts
of this section, at least one 20-ampere branch circuit shall be
provided to supply bathroom receptacle outlet(s). Such circuits shall
have no other outlets.
Exception: Where the 20-ampere circuit supplies a single bathroom, outlets for other equipment within the same bathroom shall
be permitted to be supplied in accordance with 210.23(A)(1) and
(A)(2).