The house is formed of 435 seats with one seat (one representative) per congressional district. Every seat in the House of Representatives is elected every 2 years. The number of districts varies from 1 in some states to 53 in California. The house is currently formed of 247-R, 186-D and two seats are currently vacant (previously Democratic seats). This is the most seats the Republicans have had since 1931 so it is unlikely they will improve on this number, especially a predicted general swing to the Democrats.
However, the swing as the polls stand will not be enough to give control of the House to the Democrats. However, there are some Republican seats that are vulnerable but are still expected to be held. If there is a greater swing to the Democrats, then these seats could be taken by the Democrats, thus giving them control of the House.
This election favours the Democrats as it is at the same time as the presidential election so Democratic voters are more likely to turnout when compared to midterm elections. People also tend to vote for the same party as their presidential nominee, so as the polls stand, Hillary Clinton's lead should favour the Democrats in Congress.
The numbers on the states represent the amount of predicted seats that the Republicans (red) and Democrat (blue) will get in the election.