I'd take each of your metrics and multiply it by 10, and then multiply it by another 10 for everything you haven't thought about, then probably double it for redundancy.
Because "fire temp" is meaningless in isolation. You need to know the temperature is evenly distributed (so multiple temperature probes), you need to know the temperature inside and the temperature outside (so you know your furnace isn't literally melting), you need to know it's not building pressure, you need to know it's burning as cleanly as possible (gas inflow, gas outflow, clarity of gas in, clarity of gas out, temperature of gas in, temperature of gas out, status of various gas delivery systems (fans (motor current/voltage/rpm/temp), filters, louvres, valves, pressures, flow rates)), you need to know ash is being removed correctly (that ash grates, shakers, whatever are working correctly, that ash is cooling correctly, that it's being transported away etc).
The gas out will likely go through some heat recovery stages, so you need to know gas flow through those and water flow through those. Then it will likely be scrubbed of harmful chemicals, so you need to know pressures, flow rates etc for all that.
And every motor will have voltage/current/rpm/temperature measurements. Every valve will have a commanded position and actual position. Every pipe will have pressure and temperature sensors.

The multiple fire temperature probes would then be condensed into a pertinent value and a "good" or "fault" condition for the front panel display.
The multiple air inlet would be condensed into pertinent information and a good/fault condition.
Pipes of a process will have temperature/pressure good/fault conditions (maybe a low/good/over?)

And in the old days, before microprocessors and serial communications, it would have been a local-to-sensors control/indicator panel with every reading, then a feed back to the control room where it would be "summarised". So hundreds of signals from each local control/indicator panel.

Imagine if the control room commanded a certain condition, but it wasn't being achieved because a valve was stuck or because some local control over-rode it.
How would the control room operators know where to start? Just guess?
When you see a dangerous condition building, you do what is needed to get it under control and it doesn't happen because...
You need to know why.