Scan all markets throughout the day
You need to create an excel spreadsheet that has a seperate tab for each connected market .... "Bet Angel", "Bet Angel(2)", "Bet Angel(3)" ....... through to "Bet Angel(n)" where n = the maximum number of markets that you want to connect in guardian.
Typically for Horse Racing thats usually somewhere between 20 and 50 UK/IRE races on most days but there are popular racing days like Boxing Day and New Years day where it peaks at over 80 races a day as the courses cash in on public demand for a day at the races.
For greyhounds I have no idea how many of those run a day but it is shedloads. If it maxes at say 200 markets then you would need up to "Bet Angel(200)". I'd question the point of connecting to dog races for extended periods before the off as there is pretty much no activity in most markets until quite close to the off. If you used restrict refresh on say 1800 seconds before the off you would probably get over 99.9% of the volume on dog races - just a guess on my part though.
You need to take into account how Guardian works which is basically cycling through all the connected markets. What this means is that the more markets are connected the slower the refresh rate is per connected market and that means that your bots might be too slow and miss some opportunities to take money at an absolute high or a low price.
The other thing you need to consider is what hardware you are running Bet Angel on. If you have a fairly recent PC with an i5 or i7 processor and 8GB or 16GB of ram and a SSD or NVMe hard drive then you will probably be fine. If you have got a 7 year old PC with 4gb or less of ram, a low powered celeron CPU and and a clunking old disk drive then Bet Angel and excel will probably still run with all those markets attached but your computer will struggle to keep up and slow the Guardian refresh rate down further.