Research Conclusion: Both Limits Are Structural

Online Player Limit = 255 (Protocol Bit-Width Hard Constraint)

Location	Content
src/network/network_type.h:21	static const uint MAX_CLIENTS = 255;
src/network/network_type.h:54	ClientPoolIDTag : PoolIDTraits<uint16_t, MAX_CLIENTS + 1, 0xFFFF>
src/network/core/network_game_info.cpp	clients_max / companies_max transmitted as Send_uint8 / Recv_uint8 in SerializeNetworkGameInfo (lines 251-296) and deserialization (lines 422-432)
src/table/settings/network_settings.ini:241	network.max_clients (SLE_UINT8, max = MAX_CLIENTS, default 25)
src/network/network_server.cpp:360	Acceptance check _network_clients_connected < MAX_CLIENTS

Conclusion: 255 is already the protocol field limit for uint8. To break through, clients_max and related fields must be changed to uint16 — this alters client↔server wire protocol, UDP broadcast, and the Game Coordinator protocol, which is an external protocol change (playable within a fork if both ends upgrade, but public server list compatibility is affected).

Company Limit = 15 (Tile Owner Storage Format Hard Constraint)

Location	Content
src/company_type.h:25	CompanyIDTag : PoolIDTraits<uint8_t, 0xF, 0xFF> → MAX_COMPANIES = CompanyID::End().base() = 15
src/tile_map.h:195	SetTileOwner : SB(_m[tile].m1, 0, 5, owner.base()) —— tile owner only stores 5 bits (low 5 bits of _m[].m1)
src/company_type.h:30-33	OWNER_TOWN{0x0F}, OWNER_NONE{0x10}, OWNER_WATER{0x11}, OWNER_DEITY{0x12}, OWNER_END{0x13} share the same byte space as company IDs

Conclusion: Owner and CompanyID are the same type, and the tile ownership field is only 5 bits (values 0-31), of which 15-18 are occupied by special owners. Therefore the real company limit of 15 is determined by the map array format — breaking through requires widening the tile owner storage (m1 full 8 bits or changing the _m structure) and migrating OWNER_* constants, involving full save conversion and map memory growth, which is a major format refactoring (the reason upstream OpenTTD has not done this for years).

Options

Option	Changes	Compatibility	Suggestion
A. Keep current	None	Save/Protocol/Coordinator fully compatible	✅ Recommended: 255 clients / 15 companies already far exceeds the original for multiplayer
B. Client → uint16 protocol	network_game_info.cpp serialization/deserialization change to Send_uint16/Recv_uint16; MAX_CLIENTS raised to 4095; network_settings.ini max_clients change to SLE_UINT16; settings_type.h field change to uint16; console_cmds.cpp:1056 display width	Playable within fork; public coordinator/old clients incompatible	Optional, if >255 clients is truly needed
C. Company → tile format refactor	Widen tile owner field + migrate OWNER_* constants + full save conversion	Save format change (one-time conversion)	Not recommended for short-term implementation

Delivered Content

• This design document (with exact file/line numbers);
• Optional patch option-clients-uint16.diff (complete changes for Option B, not applied, available for git apply review).

Conclusion first: "Extending" both limits is essentially a format/protocol upgrade, not a configuration adjustment; under the premise of maintaining save and public network compatibility, the current 255/15 is the upper limit. If compatibility cost is acceptable, Options B/C are as described above.