By their population they would be entitle to 6 CD's but new States have traditionally been given only one, so it's very hard to say how many Puerto Rico would have until the next apportionment.
Since 1840, 20 states have been admitted subsequent to a post-census apportionment (Kansas, Arizona, and New Mexico were included in the apportionment legislation immediately before their accession, and West Virginia took over the representation for the western part of Virginia).
13 states (FL, OR, NE, NV, CO, ND, MT, ID, WY, WA, UT, AK, and HI) were admitted with 1 representative, and kept that number after the next census. The exceptions are Washington and Hawaii. Washington had nearly quintupled in population in the decade before its admission. Based on its 1950 population, Hawaii might have been entitled to a 2nd representative, but its population was less than 1.5 of the national average. Relatively strong growth in the 1950s gave it the 2nd representative immediately after statehood. Incidentally, the 1960 Hawaii population was just barely greater than North Dakota. It was in 1970 that ND lost its 2nd representative.
6 states (TX, IA, WI, CA, MN, and SD) were admitted with 2 representatives. Only Wisconsin increased its representation at the next census. Several of these states were admitted towards the end of the decade, so the extra representative would have been based on an estimate of population. Texas and California were admitted before a US census had been held.
1 state, Oklahoma, was admitted with 5 representatives, which increased to 8 at the next census.
So the precedent would appear to be give a new state its full entitlement of representatives. An issue might be whether the House would revert back to 435 members at the next census as was done in 1960. In 2000, this would have cost Georgia, Florida, California, and North Carolina one of their new representatives, but taken a representative away from Ohio and Iowa.
