The right answer is 60 at the moment, as answered in:
Max lifetime of transactions in mempool?
This value is set by the protocol-dependent constant max_operations_ttl in:
Thus, it can eventually change on future versions of the protocol.
Notice also that 60 is an ...
NB: After the adoption of the Granada protocol, which halves the minimal time between blocks, the max_operation_ttl constant was doubled from 60 to 120 blocks, to roughly preserve the length of the live blocks and operations window when measured in wall-clock time.
The best way to send multiple transactions from a single account is grouping them into a single operation. Note that only manager operations (transaction, delegation, origination, reveal) can be grouped into a batch.
Here is an example (see more in Netezos docs):
var counter = await rpc.Blocks.Head
Both transactions sent from your address have the same fee 0.01 tez that is not enough, taking into account the gas limit 50000. Try to set the fee 0.015 tez. If this doesn't help, it also can be a problem with the node you use, so try to use another one, like Tezos Giga Node.
And yes, operations stay in the mempool for only an hour, however TzKT explorer ...
Don't worry, your funds are safe. You need to wait a bit and then try again with a higher fee or lower gas_limit.
Why did that happen?
You sent a transaction with large gas_limit: 50000 (despite a simple tz-to-tz transfer consumes about 1427 gas) and rather small, for such gas limit, fee:10000. So no one baker picked your transaction to include into a block ...