A practical maximum is a restriction on what is achievable with the resources available. Obviously as Steam gets new badges, cards, and other resources, and as badge values change, these numbers will fluctuate. Referencing various steam accounts and other information to determine Game collector badge values and this handy chart in addition to those accounts for the others. Here are the resources available:
Years of Service Badge XP: Steam was started September 12, 2003 and this badge grants 1 level worth 50 XP per year of service so the maximum at this point in time is level 10 and therefore 500 XP.
Game Collector Badge XP: Steam search lists a total of 2179 "games" available for purchase at this time including TF2(which is F2P, but lists with the $20 items), a greenlight submission fee, Multiple editions of the same game (which you can only own 1 of at a time), and DLC(which apparently need their categories fixed?), but not some games that were obtainable at some point in the past. This number becomes unreliable because of all the exceptions and the difficulty of determining what counts towards the badge. Until there's a more accurate way of determining exactly how many games someone could maximally ever own on Steam that count towards the badge, we'll go with 2041 games because we can confirm that many count towards the badge and this list should be the most current - It's a small enough difference from the search results for now that it shouldn't affect the math terribly, but in the future, it may be wise to identify the discrepancies with the search results for more accurate numbers. All badges above 250 grant 250+#ofGames XP so 2291 XP
Trading Card Badge XP: Each trading card set provides for up to 5 levels, each worth 100 XP per regular badge, and 1 level worth 100 XP per foil badge, meaning that each set of trading cards is worth 600 XP. There are currently 315 games released with Steam trading cards (results list 329, but there are Early Access duplicated here because they have current release dates), 21 Early Access games with Steam trading cards. There is also the summer getaway sale that had trading cards. Sadly, Steam search is erroneously filtering out 3 items (Wake, Lunnye Devitsy, and Dead Island) based on their release date. Even worse, not all games with Steam trading cards appear properly with Steam's search filters - 9 more games (7 Grand Steps: Step 1, Day of Defeat:Source, Eversion, Guns of Icarus Online, Jagged Alliance, La-Mulana, Offspring Fling!, Rome: Total War, and Stacking) are not properly pulling up in the trading card category, despite having trading cards. For these 349 items, the total is 209400 XP
Community Ambassador Badge XP: With a maximum level of 2, with each level worth 100 XP, this offers 200 XP.
Trading Card Beta Badge XP: Is worth 100 XP
Steam Community Translator Badge XP: is worth 100 XP
Steam Hardware Beta Candidate Badge XP: is worth 150 XP
Steamworks Community Developer Badge XP: is worth 500 XP
Steam Community Moderator Badge XP: is worth 500 XP
Valve Employee Badge XP: is worth 1000 XP
Steam Holiday Sale 2012 Badge XP: is worth a maximum of 100 XP. Value per task is used here.
Steam Summer Sale 2012 Badge XP: is worth a maximum of 100 XP. Value per task is used here.
Steam Holiday Sale 2011 Badge XP: is worth a maximum of 200 XP. Value per task is used here.
Steam Summer Camp Badge XP: is worth a maximum of 200 XP. Value per task is used here.
The Potato Sack Badge XP: is worth a maximum of 200 XP. Value per task is used here.
The Great Steam Treasure Hunt Badge XP: is worth a maximum of 100 XP. Value per task is used here.
This gives us a grand total of 500+2291+209400+200*4+100*5+150+500*2+1000 =
This is probably the long way to calculate it, but using the arithmetic progression formula for every 10 levels to determine the number of times the experience required for the first 10 levels (1000) needs to be multiplied, taking a look at level 200 gives us (20/2)*(2+(20-1)) = 210 and 210 * 1000 XP = 210000 XP required to get to level 200 and each level after that cost 2100. This leaves us at level 200 with 5641 XP remaining. 5641/2100 = 2 r1441, meaning level 202 is the maximum that gives us.
The practical maximum is somewhere around level 202. Valve employees are unlikely to have many of the community participation badges, etc. so not being a Valve employee is likely to net more points in reality. This means 1000 less from what's available and leaves the total available XP still above the threshold for level 202, meaning that the current practical maximum Steam level is 202.