My partner and I are unmarried and we have two children. How do we count our household size and income when we apply for subsidies in the Marketplace? Can we buy one policy to cover the whole family?

Assuming you are eligible for premium tax credits, the amount of your credit will be calculated based on how you file your taxes. If for example, you each claim one of your children, you each will be considered as a household of two. The income of each household would be evaluated separately to calculate eligibility for and the amount of premium tax credits and cost-sharing reductions.  Using a different example, if you claim both children as dependents on your tax return, then you and your children will be considered a household of 3, your income will be the basis for determining subsidy eligibility for the 3 of you. Your partner will be a household of one and his/her eligibility for premium tax credits will be determined separately. As for the type of coverage your family can purchase, that may vary based on the Marketplace rules where you live. For example, some insurers may offer family coverage only to married couples. If you buy one policy for the entire family, all the tax credits you are eligible for can be used to reduce the premium for that policy. If you buy separate policies, you can allocate the premium tax credits across two plans.