Well, unless you're in a rent controlled or rent stabilized dwelling..and I really don't think a townhouse in Gaithersburg would qualify -- a landlord can theoretically charge whatever increase he/she wants. As a practical matter, however, your landlord would be wise to limit any increase if your landlord knows you are good paying and responsible tenants.
DISCLAIMER: Brandy A. Peeples is licensed to practice law in the State of Maryland. This answer is being provided for informational purposes only and the laws of your jurisdiction may differ. This answer based on general legal principles and is not intended for the purpose of providing specific legal advice or opinions. Under no circumstances does this answer constitute the establishment of an attorney-client relationship. For legal advice relating to your specific situation, I strongly urge you to consult with an attorney in your area. NO COMMUNICATIONS WITH ME ARE TO BE CONSTRUED AS ARISING FROM AN ATTORNEY-CLIENT RELATIONSHIP AND NO ATTORNEY-CLIENT RELATIONSHIP WILL BE ESTABLISHED WITH ME UNLESS I HAVE EXPRESSLY AGREED TO UNDERTAKE YOUR REPRESENTATION, WHICH INCLUDES THE EXECUTION OF A WRITTEN AGREEMENT OF RETAINER.
"Fair" in a capitalist society is the amount two adults agree is fair. Unless you live in a rent-controlled building, which is highly unlikely on these facts, you and the landlord define fair. For guidance, you might check the local market. Pick up rental brochures at the entrance to many grocery stores, or check ads wherever you might look for rental accommodations. Check the cost of a place in the same general location or equivalent with the same number of bedrooms, bathrooms, other rooms, parking spaces, and other amenities. It'll be easier if you find a cooperative Realtor(r) and check through the MRIS database. Use that as a basis for your negotiations.
Reading an answer on the Internet does not create an attorney-client relationship. You are represented by me when we have both signed a retainer agreement (on paper or electronically) and some money has changed hands. Usually, you will have been asked specific questions about your situation and all potential conflicts of interest will have been resolved. Until then, you have no more right to rely on this answer than if you read it in a novel.