In our league teams are limited to seven subs, but there is no requirement that any of them are STE.
http://images.leaguerepublic.com/data/documents/616512907-league_rules_201415.pdf
Most clubs play league games with fewer than seven subs - many don't have that many players, also there is a feeling that if you bring young kids out for a game, then you really should give them some time on the field, and for a league game to give meaninful game time to all seven subs would mean weakening the team unnecessarily. Better to play with three or four subs, and plan for everyone to play at least one half (interchanges are unlimited)
Most of the time you will find one or more STE subs, but not always. It's often one. Sometimes 2, sometimes none.
Of course the majority of games are friendlies. In a friendly obviously there is no rules about STE replacemments, and the RFU Regualtion is that in a friendly you can have unlimited replacements and interchanges, and you will often see a lot of coming and going, depending on nature of the game. Sometimes you might see the weaker squad members starting and the stronger ones being left as subs (balancing the league games)