[donk.]

18btu/sqft general rule for cooling, 35btu/sqft for heating.

18x900=16K BTU required.

Variable by windows, insulation, outdoor temp, etc.

You can easily cool down all rooms with one or two units, note, the room that the unit is inside will be much colder than the rest, as the thermostat registers return temperature at the unit.
And not temperature in Lucy's room across the hall.

Portable unit may actually cost you more, and be less effective.
More effective, individual window shaker units. They are only 100$ a piece for basic units brand new.