I am trying to project all the future destroyers into available or empty pennant numbers into existing destroyer squadrons. This will tell you how far you can add destroyers without adding new destroyer squadrons. I added a note for what each squadron are best known for.
1st Destroyer Squadron --- NSF
Pennant #110 to #119?
Best known for #112 Harbin and #113 Qingdao which are Type 052; #115 and #116 which are Type 051C. 117 Xining which was XJP's parade ship during the 70th anniversary celebrations belongs to this formation and recently held exercises with the Iranian Navy.
2nd Destroyer Squadron --- SSF
Pennant #160 to #169
Best known for #167 Shenzhen which is the Type 051B, #168 Guangzhou and #169 which are Type 052B. This squadron used to be the jewel of the SSF and the PLAN but has fallen into neglect when it comes to new ships. Got its first Type 052D with the 161 Hohhot last year, which is a 052D of the second batch. Successive ships will be 052DL.
3rd Destroyer Squadron --- ESF
Pennant #130 to #139
Best known for having four Sovremennys --- 136, 137, 138 and 139. Recently #131, which is a second batch 052D, made a trip to Japan for the Japanese navy celebrations in Yokosuka.
6th Destroyer Squadron --- ESF
#150 to #159?
Probably one of the busiest squadrons as they are all over the place. Best known for having four of the six Type 052C dumped into their laps. I do not know why 052C are not distributed in a more regular fashion so don't ask me. But concentrating one type in a squadron helps with logistics like the 3rd getting all the four Project 956. Recently added #156 Zibo in their ranks, which is the first Type 052DL to be in service.
9th Destroyer Squadron --- SSF
#170 to #179?
They took the crown from the 2nd as the darling of the SSF. They got the first two Type 052C, and then a quick succession of the first batch of 052D. But after #175, the squadron got a drought of new ships. #168 and #169 used to belong to the 9th but was transferred to the 2nd. This squadron has among the most photographed ships.
10th Destroyer Squadron --- NSF
#119 or #120? to #129?
I am not completely sure where the number for the 1st and the 10th Squadron begins and ends, so correct me, and subject to correction. I am not sure if #119 Guiyang or #120 Chengdu belongs to the 1st or the 10th.
This formation has a bit of infamy for being the "old man fleet" with all the Ludas that come to roost here. As the Ludas are retired, this formation is has been getting 052D, lately and is projected to get 052DL, so it may have the last laugh. This formation has been getting 2nd batch 052D and projected to get 052DL. Most of Dalian's output of 052D seems to be going here.
101 Nanchang throws a wench in the order of the destroyer pennants. The first digit "1" still signifies destroyer but which precise unit does it belong to? It is currently stationed in Qingdao which is known for being the base of the 1st Destroyer Squadron. I do not know if further 055 will take up a more regular destroyer pennant number or will continue to use "10X"
If all these pennant slots are full, that would be around 60 destroyers. 70 if 055s fill up 101 to 110.