After the low-turnout, high-pro-statehood referendum in Puerto Rico last weekend, despite the low likelihood of it becoming a state, it's worth considering the impact that statehood might have in representation and elections.
Puerto Rico would receive two Senators, increasing the size of the Senate to 102.
Census estimates project that Puerto Rico would send five members to the House. Since 1929, the House has not expanded in size, so it would mean that Puerto Rico's delegation would come at the expense of other states' delegations. In 1959, however, with the admission of Hawaii and Alaska, Congress temporarily increased in size from 435 members to 437, then dropped back down to 435 after the 1960 Census and reapportionment. Congress might do something similar with Puerto Rico upon statehood. (For some thoughts about doubling the size of the House, see my post on the Electoral College.)
Based on projections for 2020, Puerto Rico's five seats would likely come at the expense of one seat each from California, Montana, New York, Pennsylvania, and Texas. (It's worth noting these are based on the 2020 projections; Montana is likely to receive a second representative after the 2020 reapportionment.)
This would also mean that in presidential elections, Puerto Rico would have 7 electoral votes, and these five states would each lose an electoral vote. The electoral vote total would be 540, and it would take 271 votes to win.