diff --git a/change/react-native-windows-ecffcdea-9308-4776-838d-e7e504c267da.json b/change/react-native-windows-ecffcdea-9308-4776-838d-e7e504c267da.json
new file mode 100644
index 00000000000..280b4dba89e
--- /dev/null
+++ b/change/react-native-windows-ecffcdea-9308-4776-838d-e7e504c267da.json
@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "Test HttpServer build fix (#12001)",
+  "packageName": "react-native-windows",
+  "email": "julio.rocha@microsoft.com",
+  "dependentChangeType": "patch"
+}
diff --git a/vnext/Microsoft.ReactNative/CoreApp.cpp b/vnext/Microsoft.ReactNative/CoreApp.cpp
index 4f0329c4e02..1f4eb5b5e10 100644
--- a/vnext/Microsoft.ReactNative/CoreApp.cpp
+++ b/vnext/Microsoft.ReactNative/CoreApp.cpp
@@ -231,7 +231,7 @@ static void SetProperties(JsonObject const &json, RNCoreApp *app) {
       std::wstring nsName;
       std::wstring localName;
       if (lastDot != name.rend()) {
-        nsName = std::wstring{name.begin(), static_cast<uint64_t>(name.rend() - lastDot - 1)};
+        nsName = std::wstring{name.begin(), static_cast<size_t>(name.rend() - lastDot - 1)};
         localName = std::wstring{lastDot.base()};
       } else {
         localName = name;
diff --git a/vnext/Test/HttpServer.cpp b/vnext/Test/HttpServer.cpp
index d6ca802d3cb..b08ccacf14b 100644
--- a/vnext/Test/HttpServer.cpp
+++ b/vnext/Test/HttpServer.cpp
@@ -283,8 +283,8 @@ void HttpSession::Close()
 
 #pragma region HttpServer
 
-HttpServer::HttpServer(string &&address, uint16_t port, size_t concurrency)
-  : m_ioThreadCount{concurrency}
+HttpServer::HttpServer(string &&address, uint16_t port, int concurrency)
+  : m_ioThreadCount{ static_cast<size_t>(concurrency) }
   , m_ioContext{concurrency}
   , m_acceptor{make_strand(m_ioContext)}
 {
@@ -319,7 +319,7 @@ HttpServer::HttpServer(string &&address, uint16_t port, size_t concurrency)
   }
 }
 
-HttpServer::HttpServer(uint16_t port, size_t concurrency)
+HttpServer::HttpServer(uint16_t port, int concurrency)
   : HttpServer("0.0.0.0", port, concurrency)
 {
 }
diff --git a/vnext/Test/HttpServer.h b/vnext/Test/HttpServer.h
index a3e25b2f9ad..6ba24c18082 100644
--- a/vnext/Test/HttpServer.h
+++ b/vnext/Test/HttpServer.h
@@ -126,8 +126,8 @@ class HttpServer : public std::enable_shared_from_this<HttpServer>
   // address - Valid IP address string (i.e. "127.0.0.1).
   // port    - TCP port number (i.e. 80).
   ///
-  HttpServer(std::string &&address, uint16_t port, size_t concurrency = 1);
-  HttpServer(uint16_t port, size_t concurrency = 1);
+  HttpServer(std::string &&address, uint16_t port, int concurrency = 1);
+  HttpServer(uint16_t port, int concurrency = 1);
 
   ~HttpServer();