There are two archive formats and each has it's own limit.
The total number of archives is not relevant to the limit.

• General: 256 — Exceeding this results in a crash at the main menu.
• Texture: 255 — Exceeding this causes the game to load the wrong textures for objects.

There are two types of modules (also known as plugins): Full and Light.
Any module with the .esl file name extension or the ESL flag set in its file header is a Light Module.
This can include .esm and .esp files.
All other modules (i.e. .esm and .esp without the ESL flag) are Full Modules.

• Full: 254
• Light: 4096

Fallout 4 supports two Module HEDR versions: 0.95 & 1.00.
With v1.00 the maximum number of new records was increased by allowing use of Form IDs in the extended range, doubling the capacity for new records in ESL-flagged files.
In v0.95, Form IDs in the extended range were treated as overrides of hard-coded records in Fallout4.exe.
Overrides of records from other modules do not contribute to this limit.

• Extended Range: 001-7FF (2047 IDs; 000 is the TES4 Header Record)
• Light Module v0.95: 800-FFF — 2048 New Records
• Light Module v1.00: 001-FFF — 4095 New Records
• Full Module v0.95: 800-FFF — 16,775,168 New Records
• Full Module v1.00: 001-FFF — 16,777,215 New Records