How many numbers of fire alarm devices can be connected in a single loop?

Number of devices that can be mounted in single loop may vary depending upon manufacturer of Addressable & Conventional Fire Alarm Panels.  

It depends on the brand of FACP we use. We use a range of Fire alarm panels whose maximum loop devices starts from 126 to 255.

Since it's about number of detectors in loop, so we are discussing about Addressable FAS. It does varies from make to make. 

For eg. : If we talk about BOSCH Addressable FAS (EN-54), then both FPA-5000 & FPA-1200 panels supports two type of loop modules namely LSN-300 & LSN-1500. Both the modules support maximum 254 improved devices & 127 Classic devices.

But there are other parameters like loop length & current consumption of devices that should be considered before deciding number of devices in loop.

LSN-300 can support maximum of 1600m loop length & 300mA current in loop.

LSN-1500 supports maximum 3000m loop length & 1500mA current in loop.

All the three factors are mutually linked to each other & varies inversely. Thus if loop length increases, the maximum number of detector in loop decreases & vice versa.

Special designing software (FSD) is there from BOSCH to design appropriately.

I am working on Honeywell panel it connected 180 device in single Loop. But it may depends on manufacture of Addresable and Conventional FACP.

eight devices of smoke detector and heat detector  and it defend the fire alarm system used

In ESSAR IQ8 per loop 127 outstation and 32 TR can be connected. Also it can lay the cable up to 3.5 KM distances maximum.

In GENT Vigilion, 34K Addressable systems 200 oustations including IFU & SOUNDERS in can be connected in one loop. Aso can lay the cable up to 2KM distance.

It depends on the brand name, 

but usually 254 devices (whatever it is detector or module ) per loop 

some brands and models support 254 no for a detector and 254 module per loop 

so you shall check panel datasheet

i had working on notifier panel it charged by device of fire and gas / module 

254 for improved devices and 127 for classic devices can be connected in one single loop

It is according to the company spec, max length of cable, current consumption etc