Steps for others to Reproduce
Not sure how to reproduce. This is happening in production app and is detected in Crashlytics logs.

From Crashlytics:
realm::DB::get_number_of_versions()
Crashed: com.facebook.react.JavaScript
0 app_name 0x101151344 realm::DB::get_number_of_versions() + 632572
1 app_name 0x100c46d00 check_can_create_write_transaction(realm::Realm const*) + 494 (shared_realm.cpp:494)
2 app_name 0x100c46a30 realm::Realm::begin_transaction() + 599 (shared_realm.cpp:599)
3 app_name 0x100c006a0 realm::js::RealmClass<realm::jsc::Types>::write(OpaqueJSContext const_, OpaqueJSValue_, realm::js::Arguments<realm::jsc::Types>&, realm::js::ReturnValue<realm::jsc::Types>&) + 28 (jsc_function.hpp:28)
4 app_name 0x100bfa950 OpaqueJSValue const* realm::js::wrap<&(realm::js::RealmClass<realm::jsc::Types>::write(OpaqueJSContext const_, OpaqueJSValue_, realm::js::Arguments<realm::jsc::Types>&, realm::js::ReturnValue<realm::jsc::Types>&))>(OpaqueJSContext const_, OpaqueJSValue_, OpaqueJSValue_, unsigned long, OpaqueJSValue const_ const_, OpaqueJSValue const_*) + 77 (jsc_return_value.hpp:77)
5 JavaScriptCore 0x199991d3c long long JSC::APICallbackFunction::call<JSC::JSCallbackFunction>(JSC::JSGlobalObject_, JSC::CallFrame_) + 412

react native version - 0.63.3
realm version - 10.1.0
xcode version - 12.3
pod version - 1.10.1

based on crashlytics -
majority of the crashes seen in iOS 14 (97% of the total crashes)
98% of this crash occurs in the background

complete crash log attached -
get_number_of_versions() complete crash log.txt